v0.2.0 - Thread-Local Shard Affinity

Release v0.2.0 - Thread-Local Shard Affinity

Major performance improvements through architectural changes:

Performance Gains:
- Multi-threaded throughput: 32 TiB/s at 8 threads (14-38% improvement)
- 2 threads: 14.2 TiB/s (38% faster than v0.1.5)
- 4 threads: 25.0 TiB/s (34% faster than v0.1.5)
- 8 threads: 32.0 TiB/s (14% faster than v0.1.5)
- 8.2x faster than sharded-slab in multi-threaded scenarios

Architecture Changes:
- Thread-local shard affinity replacing round-robin selection
- Each thread consistently uses same shard for better cache locality
- Reduced CPU cache line bouncing between cores
- Hash-based thread-to-shard assignment for balanced distribution

Code Quality Improvements:
- Added debug assertions to all unsafe code blocks
- Removed unnecessary const qualifiers from builder methods
- Simplified conditionals using idiomatic Rust patterns (match, let-else)
- Optimized TLS cache initialization
- Better inline attributes for compiler optimization
- Builder validation with documented panic conditions

Breaking Changes:
- None - fully backward compatible API

Benchmarks run on Intel i9-10900K @ 3.7GHz, Linux 6.14.0