docs: Add comprehensive row overflow design documentation

- Add docs/overflow/README.md: Overview and quick start guide

- Add docs/overflow/DESIGN.md: Complete technical specification

- Add docs/overflow/COMPRESSION_ANALYSIS.md: Algorithm comparison (Brotli recommended)

- Add docs/overflow/IMPLEMENTATION_GUIDE.md: 6-phase implementation plan (8 days total)

Key Features:

- SQLite-like overflow with modern improvements

- Configurable threshold (default 75% of page size)

- Doubly-linked chains for bi-directional traversal

- Optional Brotli compression (5-7% better than Gzip)

- Integration with FreeSpaceManager (O(log n) allocation)

- WAL-aware crash recovery

- Backward compatible (feature flag in header)

Target: Phase 5 implementation

Author: MPCoreDeveloper