F6 §6.2/§6.3, INV-19. One JSON per docPath under VS Code workspace storage,
never the repo. Mirrors CoauthorStore; unit-tested round-trip/paths/overwrite.
Also lands the F6 implementation plan
(docs/superpowers/plans/2026-06-11-f6-diff-view-toggle.md).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>