feat(core): add unified search loading controller

[pringelmann]

• Resolves: Loading/order algorithm + cancellation + pagination #61654

Summary

Adds a standalone controller for unified search's loading and ordering, pulled out of the UI so it can be unit-tested on its own. Nothing consumes it yet; the wiring is a follow-up.

Implements the algorithm described in #60241.

What it does

• Per-category state machine: loading / loaded / blocked / failed.
• Renders results in priority order. A lower-priority category stays blocked until the ones above it arrive, or a 1500ms timer reveals it, so results stop reshuffling on screen once they're shown.
• Cancels in-flight requests when a new search starts and drops stale responses.
• Cursor-based pagination per category. A failed page fetch keeps the results already shown and flags itself so it can be retried.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit) are included
• Screenshots before/after for front-end changes (no visual changes)
• Documentation has been updated or is not required
• Backports requested where applicable
• Labels added where applicable
• Milestone added for target branch/version

AI

• The content of this PR was partly generated using AI (jsdocs, test scaffolding and specs. All reviewed by me)

[pringelmann]

Small cleanup since the param docs were copy-paste stubs. No behaviour change.

[susnux]

looks good