Loading/order algorithm + cancellation + pagination

[pringelmann]

Replace find()/updateResults() with a dedicated search controller module implementing the issue's per-category state machine (loading / loaded / blocked / failed) with the ~1500ms reorder, so results stop reshuffling on screen. Pin the preferred order up front and fill slots in place. Apps-first ordering is already in PR #60827; review the rest of the order here.

• results don't reorder once rendered (state machine + 1500ms promote/demote)
• loading ends when all non-blocked providers finish, not the first
• a new query cancels in-flight requests and ignores stale responses
• load-more / "More from" appends and respects the server cursor / isPaginated
• dev toggle to delay/fail a provider (to demo the blocked/failed states)