fix(observer): eager archive hydration on panel open + 200-frame pages

[wpfleger96]

Problem

Opening the Activity panel for an agent/channel showed only the live in-memory window, wiped on every app restart. Archived history required 18+ manual scroll gestures to reach session start for a typical code-review turn (~900 frames). In practice, archived history was invisible.

Root cause: the archive write path is healthy; the read path never loads eagerly.

Changes

A — eager initial hydration (useObserverEvents.ts, archivePagingState.ts): on panel mount (or channel switch), automatically page through up to 10 pages (2,000 frames) before any user gesture.

B — page size 50 → 200: observer frames are small; 200/page means the 10-page budget covers 2,000 frames.

C — post-ingest channel token check: fetchOlderArchived now rechecks requestGeneration === ps.resetGeneration after await ingestArchivedObserverEvents() (async decrypt), not only after the Tauri read. Without this, a stale channel-A call resuming post-ingest could mark channel B exhausted and stop B's loop after one page.

D — generation-aware isFetching clear: the finally block only releases ps.isFetching when requestGeneration === ps.resetGeneration. Stale requests cannot steal the current channel's lock.

E — resetGeneration counter replaces channel-string ownership: ArchivePagingState now carries a monotonically increasing resetGeneration incremented by every applyChannelReset(). Each fetchOlderArchived invocation snapshots the generation at entry and rechecks it after all three async boundaries (backfill, Tauri read, ingest). A→B→A produces gen 1→2→3; old-A (gen=1) is rejected at every check regardless of channel name. activeChannelId is retained as a diagnostic field only.

F — post-backfill isFetching recheck (TOCTOU fix): ps.isFetching was checked only before await ps.backfillPromise. Two same-generation callers could both observe the lock free, both suspend on the pending backfill, then both resume past the post-backfill guard (which previously checked only generation + exhaustion) and both issue concurrent reads from the same cursor. Fix: ps.isFetching is now also included in the post-backfill guard, closing the check-then-await window.

Correctness audit

All async boundaries in fetchOlderArchived are guarded:

• After await ps.backfillPromise: generation + exhaustion + ps.isFetching ✓
• After await readArchivedObserverEventsForChannel: generation ✓ (lock held, no concurrent caller)
• After await ingestArchivedObserverEvents: generation ✓ (lock held, no concurrent caller)
• finally: generation-gated isFetching release ✓

No further TOCTOU exists: once the lock is acquired at the single acquisition point (after the post-backfill guard), all subsequent awaits are under the lock.

Tests

useLoadArchivedObserverEvents.test.mjs — four mounted-hook regression tests (real useLoadArchivedObserverEvents hook + QueryClientProvider + Tauri IPC interception):

• (a) exhausted-A → switch-B: B reads from null cursor and ingests. Green at dfb2d03; would be red at the pre-round-1 head (884ed9b).
• (b) A→B deferred-I/O race: deferred A decrypt + deferred B first read — A cannot mark B exhausted or steal B's lock. Red at dfb2d03 (bCallCount==1). Lock theft independently detected by lock probe.
• (c) concurrent backfill race: two callers both suspend on pending backfill — exactly one must acquire the lock and issue the Tauri read. Red at 4c92a01 (archiveCallCount==2). Mutation check: removing the post-backfill ps.isFetching recheck → red.
• (d) A→B→A: old-A's deferred decrypt cannot corrupt fresh-A's exhaustion state or lock. Red when resetGeneration is replaced with channel-string equality.

archivePagingState.ts unit tests cover runHydrationLoop and applyChannelReset as pure functions.

Files changed

• desktop/src/features/agents/ui/archivePagingState.ts — resetGeneration field, runHydrationLoop export, updated applyChannelReset
• desktop/src/features/agents/ui/useObserverEvents.ts — stable fetchOlderArchived (ref reads), generation token at all async boundaries, post-backfill lock recheck, generation-gated isFetching clear, hydration effect
• desktop/src/features/agents/ui/useLoadArchivedObserverEvents.test.mjs — four mounted-hook regression tests (new file)
• desktop/src/features/agents/ingestArchivedObserverEvents.test.mjs — runHydrationLoop unit tests