Adds diff::item_diff (similar), MarketplaceManager with snapshots, persisted sync reports, the gh-login slot and a repo lock held across fetches (pre-flight F11a), refresh_marketplace, load_cached_snapshot, compute_updates, pins_by_marketplace, head_for, and the GitFixture test helper on top of git::test_support (F3). AppState gains marketplace. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>