feat(workstream-c): add cheatsheet categorizer and grouping

- Implement categorize_cheatsheet() with 29-label controlled taxonomy
- Implement group_cheatsheets() with stable sha256-based group IDs
- Deterministic keyword/rule baseline, no LLM dependency
- LLM-optional path with safe fallback on failure
- Validate all CheatsheetRecord fields in __post_init__
- 50 tests covering all acceptance criteria from RFC Issue C

CheatsheetRecord uses local stub pending Workstream B merge.

Author: shreeshtripurwarcomp23-coder