docs: Add Phase 2 implementation summary and status

Documents the actual implementation completed from Phase 2 Advanced
Optimization Plan. Covers profiling infrastructure, intelligent caching,
and data structure optimizations.

SUMMARY OF COMPLETED WORK:
- Track 1 (Profiling): COMPLETE - Full infrastructure and test suite
- Track 3 (Data Structures): COMPLETE - Pattern Library indices
- Track 4 (Caching): COMPLETE - File hashing and AST parsing caches

DOCUMENTATION INCLUDES:
- Detailed specifications for each optimization
- Code examples and usage patterns
- Performance expectations and metrics
- Testing results (73 scanner tests, 63 pattern tests passing)
- Next steps and remaining work
- How-to guides for profiling and benchmarking

FILES: 1 new documentation file (PHASE2_IMPLEMENTATION_SUMMARY.md)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: 2 people