e008a8beb4
Extend `SKError` with `BadMagic`, `Truncated`, and `InvalidData` variants to replace `expect()` calls and unsafe slice indexing. Implement proper error chaining via `source()` and update `Display` formatting. Improve magic byte validation and serde error mapping for clearer debugging. Documents a broader roadmap for further crate hardening, including LRU eviction, concurrency, and benchmarking.