Add Week 10 Plugin System and Week 11 Control API

Week 10 - Plugin System (FR-19):
- Plugin traits: Plugin, OriginPlugin, MetadataPlugin, FormatPlugin
- NativePluginHost with libloading for dynamic loading
- WasmPluginHost (feature-gated) with wasmtime runtime
- PluginManager coordinating both hosts with version checks
- OriginInstance::watch() with WatchHandle, WatchEvent for live updates
- FormatPlugin::synthesize_header() for metadata overlay

Week 11 - Control API & Production (FR-17, FR-18, NFR-6, NFR-10):
- gRPC server with full MusicFS service (status, cache, origins, events)
- Proto extended: MountState enum, TierStats, full StatusResponse/CacheStats
- WebhookHandler with HMAC-SHA256 signing and exponential retry
- Metrics with latency histograms (p50/p95/p99) and origin health gauges
- CLI with mount, status, cache, search, origin, events, shutdown commands
- E2E player compatibility tests (mpv, VLC, file manager)
- systemd service, PKGBUILD, RPM spec for packaging

Plans added for Weeks 10-14 covering P1 features.
All 154 tests passing.

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

@@ -1 +1,15 @@
-#![allow(dead_code)]
+pub mod error;
+pub mod manager;
+pub mod native;
+pub mod traits;
+pub mod wasm;
+
+pub use error::{PluginError, Result};
+pub use manager::{PluginConfig, PluginEntry, PluginManager, WasmConfig};
+pub use native::NativePluginHost;
+pub use traits::{
+    ExternalMetadata, FormatPlugin, MetadataPlugin, MetadataQuery, MetadataQueryType,
+    OriginDirEntry, OriginHealth, OriginInstance, OriginPlugin, OriginStat, Plugin, PluginId,
+    PluginInfo, PluginType, WatchEvent, WatchHandle, PLUGIN_API_VERSION,
+};
+pub use wasm::{ResourceLimits, WasmPluginHost};