perf(desktop): stop idle-session event-health polling from re-rendering the whole shell

[Astro-Han]

Summary

useSessionEventHealthPolling re-renders the whole AppShell every 5 seconds for the entire lifetime of any open session, even when fully idle and no IPC is performed. This is the only periodic work the app does while idle.

Evidence

• apps/desktop/src/renderer/app-shell-effects.ts:533-560 arms window.setInterval(evaluate, 5_000) whenever activeId is set; deps [activeId, activeSession?.status, activeStreamingLive, hasInFlightLiveTools, activeInteraction?.requestId] are stable for an idle session, so the interval lives for the whole session.
• evaluate() unconditionally writes a fresh snapshot object { ...current, [activeId]: result.snapshot! } (app-shell-effects.ts:557-565). result.snapshot always carries a new checkedAt (renderer/session-event-health.ts:71-78).
• app-shell-session-ui-state.ts:105-116 replaceState only short-circuits on reference equality — the fresh object always passes → onChange → forceRender(). useAppShellSessionWorkspace is called inside AppShell (app-shell.tsx:267,1876), and ChatView/Composer/SessionListPanel are not memoized (only TurnView/MessageBody in packages/ui/src/chat-turn.tsx:137,301).
• The map is read nowhere for rendering: the only consumer is sessionEventHealthBySessionRef inside the polling hook itself (app-shell-effects.ts:544).

Fix

Gate evaluate itself, not just the interval: skip when !sessionExpectsEventStream(status, hasLiveActivity) && previous?.status === 'closed' (both booleans are already effect deps, so a session turning running re-arms automatically). Do not "skip unchanged writes" — the ref is only synced in replaceState (app-shell-session-ui-state.ts:109-113), so skipping writes would break the refreshRequestedAt throttle (every tick would refresh instead of the 10s cooldown).

Add a fake-timer regression test: an idle active session registers no interval and triggers no controller onChange; a running/live session still transitions to stale and refreshes after the threshold.

Severity

P1 (guaranteed, continuous, avoidable re-render while idle). The per-tick cost is bounded (a few ms), so P2 is defensible if the team prefers — the fix is the same either way.

Owner

apps/desktop renderer (app-shell-effects.ts), with sessionExpectsEventStream in @maka/core.

Acceptance

• Idle session: zero interval, zero forced renders.
• Running/live session: stale detection and refresh still work.