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/src/lib.rs

@@ -5,6 +5,7 @@ mod format_handler;
 mod format_layout;
 pub mod handlers;
 mod metadata;
+mod overlay;
 mod patterns;
 mod prefetch;
 mod tree;
@@ -16,6 +17,7 @@ pub use format_handler::{FormatError, FormatHandler, FormatHandlerRegistry};
 pub use format_layout::FormatLayout;
 pub use handlers::{FlacHandler, Id3v2Handler};
 pub use metadata::MetadataCache;
+pub use overlay::{OverlayError, OverlayReader};
 pub use patterns::{AccessContext, AccessPattern, PatternError, PatternStore};
 pub use prefetch::{PrefetchConfig, PrefetchEngine, PrefetchHandle};
 pub use tree::{