Completing Layer Documentation

Purpose: Process gaps.md files and add all missing documentation to layer files.

Input: docs/unwind/layers/{layer}/gaps.md from verification step Output: Updated section files with gaps filled

Anchor ids matter. gaps.md (when generated by the deterministic verifier) lists an Id for each missing item. Document it with the anchor-id heading format so re-verification matches and the coverage loop converges:

### <name> [MUST|SHOULD|DON'T] <!-- id: <Id from gaps.md> -->

If an item should NOT be in the rebuild, move it to an ## Excluded section with a one-line reason instead of documenting it (see analysis-principles.md 16).

Process