v0.3.0

Performance release. Rust now wins 15/23 benchmarks against C++ cvmfs2 v2.11.5.

Key optimizations:

• Interior mutability for repository catalogs (RwLock, read locks on hot path)
• Arc in lookup cache (no clone overhead)
• Readdir pre-populates lookup cache for subsequent getattr calls
• All chunk file handles cached (no re-open on chunk transitions)
• Thread-local read buffer (eliminates per-call heap allocation)
• Data objects never expire in cache (content-addressed = immutable)
• FOPEN_KEEP_CACHE for kernel file content caching
• Benchmarks rewritten with hyperfine for accurate measurement