fix(desktop): keep busy tile transcripts stable

[Dolverin]

What does this PR do?

Hardens the workspace-tile transcript reconciliation introduced by #95600 / #94255 so a persisted snapshot cannot replace a tile's live output or make unrelated tabs jump.

reconcileTileTranscripts() currently gates every tile with the main pane's global $busy state. That produces both failure directions:

• an idle tile is not reconciled while the main pane is busy;
• a busy tile is still reconciled while the main pane is idle, so a stale persisted snapshot can overwrite its live transcript.

The async read also captures tile presence before getLatestSessionMessages(), allowing a result to land after the tile becomes busy, closes, or is rebound.

Changes

• decide liveness per tile runtime from $sessionStates (busy, awaitingResponse, needsInput, or turnLive);
• check that runtime-specific liveness both before and after the async storage read;
• re-read the live stored-session/runtime pair after the await so closed or rebound tiles discard stale results;
• prune signature entries for tiles that have closed, including after a successful earlier reconciliation;
• preserve idle-tile refresh while the main pane is busy;
• leave the main-pane backfill and empty-transcript behavior unchanged.

This is complementary to #95770 (cheap no-change fetch gating) and #96215 (warm-cache refresh on explicit Bot Chat reopen); neither protects an active tile from an in-flight persisted snapshot.

Regression proof

On current origin/main / v0.20.6, with only a test-harness compatibility default for the old busyRef signature:

use-background-sync.test.ts: 5 failed, 26 passed

The five failures are the intended contracts:

1. idle tile still reconciles while the main pane is busy;
2. busy tile is skipped while the main pane is idle;
3. tile becoming busy during the read rejects the snapshot;
4. tile closing during the read rejects the snapshot;
5. closed-tile signature is pruned.

On this branch:

use-background-sync.test.ts: 31 passed
TypeScript typecheck (all three desktop tsconfigs): passed
Scoped ESLint: passed
Production desktop build + assert-dist-built: passed
git diff --check: passed

Scope

Only these files change:

• apps/desktop/src/app/contrib/hooks/use-background-sync.ts
• apps/desktop/src/app/contrib/hooks/use-background-sync.test.ts

[Enough1122]

AI code review — automated review for reference; please use your judgment.

fix(desktop): keep busy tile transcripts stable — correctly scopes tile reconciliation to per-runtime live state.

• apps/desktop/src/app/contrib/hooks/use-background-sync.ts:239 — tileRuntimeOwnsLiveState(runtimeId) checks $sessionStates.get()[runtimeId] for busy || awaitingResponse || needsInput || turnLive. Replaces prior busyRef = $busy (primary pane only), which clobbered a busy tile's streaming assistant message with a stale persisted fetch, and also blocked idle-tile background deliveries when primary was busy.
• Prunes signatureRef for closed tiles (tile:${storedId} keys not in current $sessionTiles) so map doesn't leak per ever-opened tile (fix(desktop): reconcile workspace-tile transcripts on sessions.changed #94255). Re-checks tileStillPresent() and tileRuntimeOwnsLiveState after await getLatestSessionMessages to discard racy snapshots when tile goes busy or closes mid-read.
• use-background-sync.test.ts covers: active refresh still works, idle tile reconciles while main busy, busy tile skipped, racy-busy and closing discard, signature pruning. The prior updaterCallCount>0 test now also asserts background delivery text preserved.

Approve.

[teknium1]

Thanks — this landed on main via #101214 (6d061ed) with your commit/credit preserved. Your commit was cherry-picked as-is (tests trimmed to the three distinct invariants). Closing the original in favor of the merged salvage.