Implement Week 2 metadata extraction and cache database

Week 1 fixes:
- Move hex to workspace dependencies
- Add cargo-criterion, protobuf, grpcurl to flake.nix

Week 2 implementation:
- musicfs-metadata: MetadataParser with symphonia 0.5 for FLAC, MP3,
  Opus/Vorbis, M4A/AAC (2 tests)
- musicfs-cache: SQLite schema per architecture 4.3.6 with track/disc
  columns, TEXT content_hash, all required indexes
- musicfs-cache/db.rs: Database with upsert, CRUD, mtime lookup (9 tests)
- musicfs-cache/metadata.rs: MetadataCache with store/lookup/is_fresh/
  invalidate (2 tests)
- musicfs-core: Added Metadata error variant

22 tests pass total. Oracle-verified against architecture doc.

musicfs/crates/musicfs-metadata/src/lib.rs

@@ -1 +1,3 @@
-#![allow(dead_code)]
+mod parser;
+
+pub use parser::MetadataParser;

musicfs/crates/musicfs-metadata/src/parser.rs

@@ -0,0 +1,134 @@
+use musicfs_core::{AudioFormat, AudioMeta, Error, Result};
+use std::fs::File;
+use std::path::Path;
+use symphonia::core::codecs::CODEC_TYPE_NULL;
+use symphonia::core::formats::FormatOptions;
+use symphonia::core::io::MediaSourceStream;
+use symphonia::core::meta::MetadataOptions;
+use symphonia::core::probe::Hint;
+use tracing::debug;
+
+pub struct MetadataParser;
+
+impl MetadataParser {
+    pub fn new() -> Self {
+        Self
+    }
+
+    pub fn parse_file(&self, path: &Path) -> Result<AudioMeta> {
+        let file = File::open(path)?;
+        let mss = MediaSourceStream::new(Box::new(file), Default::default());
+
+        let ext = path.extension().and_then(|e| e.to_str()).unwrap_or("");
+
+        let mut hint = Hint::new();
+        if !ext.is_empty() {
+            hint.with_extension(ext);
+        }
+
+        let fmt_opts = FormatOptions::default();
+        let meta_opts = MetadataOptions::default();
+
+        let probed = symphonia::default::get_probe()
+            .format(&hint, mss, &fmt_opts, &meta_opts)
+            .map_err(|e| Error::Metadata(format!("Failed to probe format: {}", e)))?;
+        let mut format = probed.format;
+
+        let mut audio_meta = AudioMeta {
+            format: AudioFormat::from_extension(ext),
+            ..Default::default()
+        };
+
+        if let Some(metadata) = format.metadata().current() {
+            self.extract_tags(&mut audio_meta, metadata);
+        }
+
+        if let Some(track) = format
+            .tracks()
+            .iter()
+            .find(|t| t.codec_params.codec != CODEC_TYPE_NULL)
+        {
+            let params = &track.codec_params;
+
+            if let Some(n_frames) = params.n_frames {
+                if let Some(sample_rate) = params.sample_rate {
+                    audio_meta.duration_ms =
+                        Some((n_frames as u64 * 1000) / sample_rate as u64);
+                    audio_meta.sample_rate = Some(sample_rate);
+                }
+            }
+
+            if let Some(bits_per_sample) = params.bits_per_sample {
+                if let Some(sample_rate) = params.sample_rate {
+                    if let Some(channels) = params.channels {
+                        audio_meta.bitrate = Some(
+                            bits_per_sample * sample_rate * channels.count() as u32 / 1000,
+                        );
+                    }
+                }
+            }
+        }
+
+        debug!(?audio_meta, "Parsed metadata");
+        Ok(audio_meta)
+    }
+
+    fn extract_tags(
+        &self,
+        meta: &mut AudioMeta,
+        metadata: &symphonia::core::meta::MetadataRevision,
+    ) {
+        use symphonia::core::meta::StandardTagKey;
+
+        for tag in metadata.tags() {
+            if let Some(std_key) = tag.std_key {
+                let value = tag.value.to_string();
+                match std_key {
+                    StandardTagKey::TrackTitle => meta.title = Some(value),
+                    StandardTagKey::Artist => meta.artist = Some(value),
+                    StandardTagKey::Album => meta.album = Some(value),
+                    StandardTagKey::AlbumArtist => meta.album_artist = Some(value),
+                    StandardTagKey::Genre => meta.genre = Some(value),
+                    StandardTagKey::TrackNumber => {
+                        meta.track = value.split('/').next().and_then(|s| s.parse().ok());
+                    }
+                    StandardTagKey::DiscNumber => {
+                        meta.disc = value.split('/').next().and_then(|s| s.parse().ok());
+                    }
+                    StandardTagKey::Date | StandardTagKey::ReleaseDate => {
+                        meta.year = value.chars().take(4).collect::<String>().parse().ok();
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+}
+
+impl Default for MetadataParser {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_audio_format_detection() {
+        assert_eq!(AudioFormat::from_extension("flac"), AudioFormat::Flac);
+        assert_eq!(AudioFormat::from_extension("mp3"), AudioFormat::Mp3);
+        assert_eq!(AudioFormat::from_extension("opus"), AudioFormat::Opus);
+        assert_eq!(AudioFormat::from_extension("ogg"), AudioFormat::Vorbis);
+        assert_eq!(AudioFormat::from_extension("m4a"), AudioFormat::Aac);
+        assert_eq!(AudioFormat::from_extension("wav"), AudioFormat::Wav);
+    }
+
+    #[test]
+    fn test_parser_creation() {
+        let parser = MetadataParser::new();
+        let default_parser = MetadataParser::default();
+        assert!(std::mem::size_of_val(&parser) == std::mem::size_of_val(&default_parser));
+    }
+}