feat(cache): implement OverlayReader for header/audio splice

- Implement three-region splice logic (header, audio, boundary)
- Add passthrough mode for files without format_layout
- Add estimate_virtual_size() for getattr
- Create OverlayError enum with proper error conversions
- Add 8 comprehensive unit tests
- All tests pass, LSP diagnostics clean

crates/musicfs-cache/Cargo.toml

@@ -7,6 +7,7 @@ edition.workspace = true
 musicfs-core = { path = "../musicfs-core" }
 musicfs-cas = { path = "../musicfs-cas" }
 musicfs-metadata = { path = "../musicfs-metadata" }
+bytes.workspace = true
 rusqlite = { workspace = true, features = ["bundled"] }
 sled.workspace = true
 tokio.workspace = true