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Add reference comments to vouch records master

Accept optional public comments of at most 300 characters. Preserve comments through PDS publication, ingestion, and SQLite storage. Migrate existing records with an empty comment. Attach incoming vouch comments to selected path nodes after trust calculation. Comments do not affect scores, paths, or record keys. Document the API field and treat comments as untrusted text. Go race tests, vet, schema lint, hooks, and Nix checks pass. tsk-20