Add comprehensive mechanics catalog (docs/mechanics.md) ...

Master catalog of all mechanisms in the Navi project:
- 80+ mechanics catalogued across all subsystems
- Each with description, config flags, source files, documentation status
- 50 undocumented mechanics explicitly listed
- Cross-reference index for feature planning
- Updated docs/index.md to link the catalog

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>