v2.0.18

docs: cancel write coalescing optimization plans

Phase 3.1 (lock-free unified cache) is complete and provides excellent
performance (2.8x improvement). Write coalescing is no longer needed.

Changes:
- CLAUDE.md: Remove Phase 3.2 references, mark design docs as cancelled
- README.md: Remove write coalescing from Future Improvements
- README.md: Add lock-free unified cache to Completed Improvements

Signed-off-by: Date Huang <tjjh89017@hotmail.com>