Content snapshot export

Purpose: Move committed public content and reusable score results through ordinary downloads. Scenario: A TAR or ZIP download adds an allowlisted Fastmod score supplement automatically. Changing or pruning scores changes the content archive cache identity even without a Git commit; image archives do not depend on scores. SQLite statement snapshots include committed WAL data. Missing score stores export an empty supplement without creating a database. Sources: archive builder (source unavailable), score snapshot (source unavailable), format.