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/Cargo.toml

@@ -4,3 +4,20 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
+musicfs-core = { path = "../musicfs-core" }
+async-trait.workspace = true
+tokio.workspace = true
+thiserror.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
+libloading = "0.8"
+wasmtime = { version = "19", optional = true }
+semver = "1"
+
+[features]
+default = []
+wasm = ["wasmtime"]
+
+[dev-dependencies]
+tempfile.workspace = true