Add Week 4b plan: Origin-CAS connector for cache-miss handling

- Create week-04b-origin-connector.md with ContentFetcher design
- Update development-plan.md: Phase 1 now includes Week 4b
- Update architecture.md: Phase 1 table includes Week 4b
- Plan includes EventBus integration per FR-18.1 (Oracle-verified)

docs/v2/architecture.md

@@ -1005,7 +1005,7 @@ and better fit for systems programming.
 
 ## 7. Implementation Plan
 
-### 7.1 Phase 1: MVP (4 weeks)
+### 7.1 Phase 1: MVP (4.5 weeks)
 
 **Goal:** Basic functional filesystem with single origin.
 
@@ -1014,7 +1014,8 @@ and better fit for systems programming.
 | 1 | Project setup, FUSE skeleton, local origin plugin |
 | 2 | Metadata extraction (symphonia), SQLite schema |
 | 3 | Virtual path resolver, tree cache, basic readdir/stat/read |
-| 4 | CAS implementation, chunk caching, integration tests |
+| 4 | CAS implementation, chunk caching, LRU eviction |
+| 4b | Origin→CAS connector (ContentFetcher), cache-miss handling |
 
 **Exit Criteria:**
 - Mount and browse local music library

docs/v2/development-plan.md

@@ -64,12 +64,14 @@ musicfs/
 
 ---
 
-## 2. Phase 1: MVP (Weeks 1-4)
+## 2. Phase 1: MVP (Weeks 1-4b)
 
 **Goal**: Basic functional filesystem with single local origin.
 
 **Requirements Covered**: FR-1, FR-2, FR-3, FR-4, FR-5, FR-6, FR-7, FR-8, FR-9, FR-18, NFR-1.1-1.7
 
+**Note**: Week 4b bridges Origin→CAS data flow (cache-miss handling) required for actual file reads.
+
 ---
 
 ### Week 1: Foundation
@@ -523,8 +525,55 @@ impl LruEviction {
 
 - [ ] Chunks stored in CAS with deduplication
 - [ ] Cache size limit enforced via eviction
-- [ ] Audio playback works through mounted filesystem
 - [ ] Cache persists across daemon restarts
+
+**Note**: Audio playback requires Week 4b (Origin→CAS connector).
+
+---
+
+### Week 4b: Origin-CAS Connector
+
+**Detailed plan**: See `plans/week-04b-origin-connector.md`
+
+#### Summary
+
+Bridges the gap between Origin (source files) and CAS (chunk cache). Without this, FUSE read() cannot return actual file content.
+
+#### Deliverables
+
+| Task | Crate | Files | Requirements |
+|------|-------|-------|--------------|
+| ContentFetcher | musicfs-cas | `fetcher.rs` | FR-3.2 |
+| Cache-miss handling | musicfs-cas | `reader.rs` | FR-3.2 |
+| FUSE integration | musicfs-fuse | `filesystem.rs` | FR-3.1-3.2 |
+
+#### Key Components
+
+```rust
+pub struct ContentFetcher {
+    store: Arc<CasStore>,
+    origins: HashMap<OriginId, Arc<dyn Origin>>,
+    file_meta: HashMap<FileId, FileMeta>,
+}
+
+impl ContentFetcher {
+    /// Fetch file from origin, store in CAS, return manifest
+    pub async fn fetch_file(&self, file_id: FileId) -> Result<ChunkManifest>;
+}
+```
+
+#### Tests
+
+| Test | Type | Validates |
+|------|------|-----------|
+| `test_fetch_file` | Unit | Origin → CAS works |
+| `test_reader_cache_miss` | Unit | Fetcher called on miss |
+| `test_e2e_cat_file` | E2E | `cat` returns content |
+
+#### Exit Criteria
+
+- [ ] `cat /mnt/musicfs/Artist/Album/track.flac` returns actual data
+- [ ] Audio playback works through mounted filesystem
 - [ ] All Phase 1 requirements pass acceptance tests
 
 ---

docs/v2/plans/week-04b-origin-connector.md

@@ -0,0 +1,501 @@
+# Week 4b: Origin-CAS Connector
+
+**Phase**: 1 (MVP)  
+**Prerequisites**: Week 4 (CAS & Chunk Caching)  
+**Estimated effort**: 1 day
+
+---
+
+## Objective
+
+Bridge the gap between Origin (source files) and CAS (chunk cache) to enable actual file reads through FUSE. This implements the "cache miss" flow from architecture section 4.3.5.
+
+**Problem**: Week 4 implemented CAS storage and FileReader, but there's no code that:
+1. Detects when requested chunks aren't cached
+2. Fetches data from Origin
+3. Stores chunks in CAS
+4. Creates ChunkManifest for the file
+
+**Solution**: Create `ContentFetcher` that orchestrates Origin → CAS data flow on cache miss.
+
+---
+
+## Architecture Reference
+
+From architecture.md section 4.3.5 (Read Operation Activity):
+
+```
+|CAS|
+:compute chunk range for [offset, offset+size];
+if (all chunks cached?) then (yes)
+    :read from local chunk files;
+else (no)
+    |OriginFederation|
+    :select healthy origin by priority;
+    :fetch missing byte range;
+    |CAS|
+    :chunk fetched data (CDC);
+    :store chunks by hash;
+    :update chunk manifest;
+endif
+```
+
+---
+
+## Deliverables
+
+| Task | Crate | Files | Done |
+|------|-------|-------|------|
+| ContentFetcher implementation | musicfs-cas | `fetcher.rs` | [ ] |
+| FileId → FileMeta resolver | musicfs-cas | `fetcher.rs` | [ ] |
+| Update FileReader for cache-miss | musicfs-cas | `reader.rs` | [ ] |
+| Update FUSE with fetcher | musicfs-fuse | `filesystem.rs` | [ ] |
+| E2E test: cat file through FUSE | tests | `integration.rs` | [ ] |
+
+---
+
+## Task 1: ContentFetcher
+
+### 1.1 Create `musicfs-cas/src/fetcher.rs`
+
+```rust
+use crate::{CasStore, ChunkManifest, ChunkRef};
+use musicfs_core::{Event, EventBus, FileId, FileMeta, OriginId, RealPath};
+use musicfs_origins::Origin;
+use std::collections::HashMap;
+use std::path::Path;
+use std::sync::{Arc, RwLock};
+use tracing::{debug, info};
+
+pub struct ContentFetcher {
+    store: Arc<CasStore>,
+    origins: RwLock<HashMap<OriginId, Arc<dyn Origin>>>,
+    file_meta: RwLock<HashMap<FileId, FileMeta>>,
+    event_bus: Option<Arc<EventBus>>,
+}
+
+impl ContentFetcher {
+    pub fn new(store: Arc<CasStore>) -> Self {
+        Self {
+            store,
+            origins: RwLock::new(HashMap::new()),
+            file_meta: RwLock::new(HashMap::new()),
+            event_bus: None,
+        }
+    }
+
+    pub fn with_event_bus(store: Arc<CasStore>, event_bus: Arc<EventBus>) -> Self {
+        Self {
+            store,
+            origins: RwLock::new(HashMap::new()),
+            file_meta: RwLock::new(HashMap::new()),
+            event_bus: Some(event_bus),
+        }
+    }
+
+    pub fn register_origin(&self, origin: Arc<dyn Origin>) {
+        let id = origin.id().clone();
+        self.origins.write().unwrap().insert(id, origin);
+    }
+
+    pub fn register_file(&self, meta: FileMeta) {
+        self.file_meta.write().unwrap().insert(meta.id, meta);
+    }
+
+    pub fn register_files(&self, files: impl IntoIterator<Item = FileMeta>) {
+        let mut map = self.file_meta.write().unwrap();
+        for meta in files {
+            map.insert(meta.id, meta);
+        }
+    }
+
+    pub async fn fetch_file(&self, file_id: FileId) -> Result<ChunkManifest, FetchError> {
+        let meta = {
+            let files = self.file_meta.read().unwrap();
+            files.get(&file_id).cloned()
+                .ok_or(FetchError::FileNotFound(file_id))?
+        };
+
+        let origin = {
+            let origins = self.origins.read().unwrap();
+            origins.get(&meta.real_path.origin_id).cloned()
+                .ok_or_else(|| FetchError::OriginNotFound(meta.real_path.origin_id.clone()))?
+        };
+
+        info!("Fetching file {:?} from origin {}", file_id, origin.id());
+
+        let data = origin.read(&meta.real_path.path, 0, meta.size as u32).await
+            .map_err(|e| FetchError::OriginRead(e.to_string()))?;
+
+        let hash = self.store.put(&data).await
+            .map_err(FetchError::Store)?;
+
+        let manifest = ChunkManifest {
+            file_id,
+            total_size: meta.size,
+            chunks: vec![ChunkRef {
+                hash,
+                offset: 0,
+                size: data.len() as u32,
+            }],
+        };
+
+        debug!("Created manifest for {:?}: {} bytes, 1 chunk", file_id, meta.size);
+
+        Ok(manifest)
+    }
+
+    pub fn emit_access_event(&self, meta: &FileMeta, offset: u64, size: u32) {
+        if let Some(bus) = &self.event_bus {
+            bus.publish(Event::FileAccessed {
+                path: meta.virtual_path.clone(),
+                origin_id: meta.real_path.origin_id.clone(),
+                offset,
+                size,
+            });
+        }
+    }
+
+    pub async fn ensure_cached(&self, file_id: FileId) -> Result<ChunkManifest, FetchError> {
+        self.fetch_file(file_id).await
+    }
+
+    pub fn get_file_meta(&self, file_id: FileId) -> Option<FileMeta> {
+        self.file_meta.read().unwrap().get(&file_id).cloned()
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum FetchError {
+    #[error("File not found: {0:?}")]
+    FileNotFound(FileId),
+
+    #[error("Origin not found: {0}")]
+    OriginNotFound(OriginId),
+
+    #[error("Origin read error: {0}")]
+    OriginRead(String),
+
+    #[error("Store error: {0}")]
+    Store(#[from] crate::CasError),
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::CasConfig;
+    use musicfs_core::VirtualPath;
+    use musicfs_origins::LocalOrigin;
+    use std::path::PathBuf;
+    use std::time::SystemTime;
+    use tempfile::TempDir;
+
+    #[tokio::test]
+    async fn test_fetch_file() {
+        let cas_dir = TempDir::new().unwrap();
+        let origin_dir = TempDir::new().unwrap();
+
+        std::fs::write(origin_dir.path().join("test.flac"), b"fake audio data").unwrap();
+
+        let config = CasConfig {
+            chunks_dir: cas_dir.path().join("chunks"),
+            ..Default::default()
+        };
+        let store = Arc::new(CasStore::open(config).await.unwrap());
+        let fetcher = ContentFetcher::new(store.clone());
+
+        let origin = Arc::new(LocalOrigin::new("local", origin_dir.path()));
+        fetcher.register_origin(origin);
+
+        let meta = FileMeta {
+            id: FileId(1),
+            virtual_path: VirtualPath::new("/Artist/Album/test.flac"),
+            real_path: RealPath {
+                origin_id: OriginId::from("local"),
+                path: PathBuf::from("/test.flac"),
+            },
+            size: 15,
+            mtime: SystemTime::now(),
+            content_hash: None,
+            audio: None,
+        };
+        fetcher.register_file(meta);
+
+        let manifest = fetcher.fetch_file(FileId(1)).await.unwrap();
+        assert_eq!(manifest.total_size, 15);
+        assert_eq!(manifest.chunks.len(), 1);
+
+        let data = store.get(&manifest.chunks[0].hash).await.unwrap();
+        assert_eq!(&data[..], b"fake audio data");
+    }
+
+    #[tokio::test]
+    async fn test_fetch_file_not_found() {
+        let cas_dir = TempDir::new().unwrap();
+        let config = CasConfig {
+            chunks_dir: cas_dir.path().join("chunks"),
+            ..Default::default()
+        };
+        let store = Arc::new(CasStore::open(config).await.unwrap());
+        let fetcher = ContentFetcher::new(store);
+
+        let result = fetcher.fetch_file(FileId(999)).await;
+        assert!(matches!(result, Err(FetchError::FileNotFound(_))));
+    }
+
+    #[tokio::test]
+    async fn test_fetch_emits_event() {
+        let cas_dir = TempDir::new().unwrap();
+        let origin_dir = TempDir::new().unwrap();
+        std::fs::write(origin_dir.path().join("test.flac"), b"audio").unwrap();
+
+        let config = CasConfig {
+            chunks_dir: cas_dir.path().join("chunks"),
+            ..Default::default()
+        };
+        let store = Arc::new(CasStore::open(config).await.unwrap());
+        let event_bus = Arc::new(EventBus::default());
+        let mut rx = event_bus.subscribe();
+
+        let fetcher = ContentFetcher::with_event_bus(store, event_bus);
+        let origin = Arc::new(LocalOrigin::new("local", origin_dir.path()));
+        fetcher.register_origin(origin);
+
+        let meta = FileMeta {
+            id: FileId(1),
+            virtual_path: VirtualPath::new("/Artist/test.flac"),
+            real_path: RealPath {
+                origin_id: OriginId::from("local"),
+                path: PathBuf::from("/test.flac"),
+            },
+            size: 5,
+            mtime: SystemTime::now(),
+            content_hash: None,
+            audio: None,
+        };
+        fetcher.register_file(meta.clone());
+
+        fetcher.emit_access_event(&meta, 0, 5);
+
+        let event = rx.try_recv().unwrap();
+        assert!(matches!(event, Event::FileAccessed { .. }));
+    }
+}
+```
+
+---
+
+## Task 2: Update FileReader
+
+### 2.1 Update `musicfs-cas/src/reader.rs`
+
+Add fetcher integration for cache-miss handling:
+
+```rust
+use crate::fetcher::{ContentFetcher, FetchError};
+
+pub struct FileReader {
+    store: Arc<CasStore>,
+    fetcher: Option<Arc<ContentFetcher>>,
+    manifests: RwLock<HashMap<FileId, ChunkManifest>>,
+}
+
+impl FileReader {
+    pub fn new(store: Arc<CasStore>) -> Self {
+        Self {
+            store,
+            fetcher: None,
+            manifests: RwLock::new(HashMap::new()),
+        }
+    }
+
+    pub fn with_fetcher(store: Arc<CasStore>, fetcher: Arc<ContentFetcher>) -> Self {
+        Self {
+            store,
+            fetcher: Some(fetcher),
+            manifests: RwLock::new(HashMap::new()),
+        }
+    }
+
+    pub async fn read(
+        &self,
+        file_id: FileId,
+        offset: u64,
+        size: u32,
+    ) -> Result<Bytes, ReaderError> {
+        let manifest = self.get_or_fetch_manifest(file_id).await?;
+        
+        if let Some(fetcher) = &self.fetcher {
+            if let Some(meta) = fetcher.get_file_meta(file_id) {
+                fetcher.emit_access_event(&meta, offset, size);
+            }
+        }
+        
+        // ... rest of read logic unchanged
+    }
+
+    async fn get_or_fetch_manifest(&self, file_id: FileId) -> Result<ChunkManifest, ReaderError> {
+        {
+            let manifests = self.manifests.read().unwrap();
+            if let Some(m) = manifests.get(&file_id) {
+                return Ok(m.clone());
+            }
+        }
+
+        let Some(fetcher) = &self.fetcher else {
+            return Err(ReaderError::ManifestNotFound(file_id));
+        };
+
+        let manifest = fetcher.ensure_cached(file_id).await
+            .map_err(ReaderError::Fetch)?;
+
+        self.manifests.write().unwrap().insert(file_id, manifest.clone());
+        Ok(manifest)
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum ReaderError {
+    #[error("Manifest not found for file {0:?}")]
+    ManifestNotFound(FileId),
+
+    #[error("Fetch error: {0}")]
+    Fetch(#[from] FetchError),
+
+    #[error("CAS error: {0}")]
+    Cas(#[from] crate::CasError),
+}
+```
+
+---
+
+## Task 3: Update lib.rs
+
+### 3.1 Update `musicfs-cas/src/lib.rs`
+
+```rust
+mod chunks;
+mod fetcher;
+mod reader;
+mod store;
+
+pub use chunks::{ChunkLocation, ChunkRef};
+pub use fetcher::{ContentFetcher, FetchError};
+pub use reader::{ChunkManifest, FileReader, ReaderError};
+pub use store::{CasConfig, CasError, CasStore, DedupStats};
+```
+
+---
+
+## Task 4: Update Cargo.toml
+
+### 4.1 Update `musicfs-cas/Cargo.toml`
+
+```toml
+[dependencies]
+musicfs-core = { path = "../musicfs-core" }
+musicfs-origins = { path = "../musicfs-origins" }
+# ... rest unchanged
+```
+
+---
+
+## Task 5: Update FUSE Integration
+
+### 5.1 Update `musicfs-fuse/src/filesystem.rs`
+
+```rust
+use musicfs_cas::{ContentFetcher, FileReader};
+
+pub struct MusicFs {
+    tree: Arc<RwLock<VirtualTree>>,
+    reader: Option<Arc<FileReader>>,
+    fetcher: Option<Arc<ContentFetcher>>,
+    uid: u32,
+    gid: u32,
+}
+
+impl MusicFs {
+    pub fn with_content_access(
+        tree: Arc<RwLock<VirtualTree>>,
+        reader: Arc<FileReader>,
+        fetcher: Arc<ContentFetcher>,
+    ) -> Self {
+        Self {
+            tree,
+            reader: Some(reader),
+            fetcher: Some(fetcher),
+            uid: unsafe { libc::getuid() },
+            gid: unsafe { libc::getgid() },
+        }
+    }
+}
+```
+
+---
+
+## Tests
+
+| Test | Type | Validates |
+|------|------|-----------|
+| `test_fetch_file` | Unit | Origin → CAS fetch works |
+| `test_fetch_file_not_found` | Unit | Missing file error |
+| `test_fetch_emits_event` | Unit | FileAccessed event emitted (FR-18.1) |
+| `test_reader_with_fetcher` | Unit | Cache-miss triggers fetch |
+| `test_e2e_cat_file` | Integration | `cat` returns file content |
+
+---
+
+## Exit Criteria
+
+- [ ] `ContentFetcher` fetches from Origin and stores in CAS
+- [ ] `FileReader` calls fetcher on cache miss
+- [ ] File metadata (FileId → FileMeta) is resolvable
+- [ ] `cat /mnt/musicfs/Artist/Album/track.flac` returns actual audio data
+- [ ] All existing tests still pass
+
+---
+
+## Dependencies
+
+### `musicfs-cas/Cargo.toml`
+
+```toml
+[dependencies]
+musicfs-origins = { path = "../musicfs-origins" }
+```
+
+---
+
+## Implementation Notes
+
+1. **Week 4 treated whole files as single chunks** - this continues that approach
+2. **CDC chunking deferred to Week 5** - fetcher will be updated then
+3. **No OriginFederation yet** - single origin lookup for MVP
+4. **FileMeta registration** - caller must register files before they can be fetched
+5. **EventBus integration** - emits `FileAccessed` event per FR-18.1 (P0)
+6. **Full file fetch** - currently fetches entire file on cache miss; byte-range optimization deferred
+
+## Architecture Compliance
+
+| Architecture Section | Requirement | Status |
+|---------------------|-------------|--------|
+| 4.3.5 | Cache miss → fetch from origin | ✅ |
+| 4.3.5 | Store chunks by hash | ✅ |
+| 4.3.5 | Update chunk manifest | ✅ |
+| 4.3.5 | Emit FileAccessed event | ✅ |
+| 4.3.3 | OriginFederation (multi-origin) | ⏳ Deferred |
+| 4.3.5 | Byte-range fetch | ⏳ Deferred |
+| 4.3.5 | CDC chunking | ⏳ Week 5 |
+
+---
+
+## Next Steps
+
+After this, the MVP is complete:
+- Mount filesystem
+- Browse virtual tree (Artist/Album/Track)
+- Read actual file content through FUSE
+- Audio playback works
+
+Week 5 adds CDC chunking for efficient delta sync.