ItemUpdate gains invalid_at_head: compute_updates records why the item
cannot be installed at head (catalog invalid, or gone) — the rule
update_marketplace_item applies — read once per marketplace from the
snapshot at head, else from the catalog parsed at head. The Installed row
shows that reason instead of a Review button, and the update counts in the
Marketplace tab and Settings count only applicable updates.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>