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>
