fix(desktop): resolve O(n²) render cascade in activity sidebar

[wesbillman]

Summary

• Fixes the activity sidebar becoming unresponsive after 1-2 minutes when an agent is actively streaming events
• Root cause was an O(n²) render cascade: every new WebSocket event triggered a full transcript rebuild over all events and re-rendered every markdown block
• Four-layer fix: store-level incremental transcript, useSyncExternalStore with snapshot caching, component cleanup, and React.memo on transcript items

Approach

Layer 1 — Store-level incremental transcript (observerRelayStore.ts)
Transcript is now computed incrementally per event (O(1)) instead of O(n) full rebuild per render. Falls back to full rebuild for out-of-order arrivals and when the 800-event cap trims.

Layer 2 — useSyncExternalStore hooks (useObserverEvents.ts)
Replaced useState/useEffect subscription pattern. Snapshot caching gives referential stability — React only re-renders when the snapshot reference actually changes.

Layer 3 — Component cleanup (ManagedAgentSessionPanel.tsx)
Removed buildTranscript useMemo and the [...events].reverse().find() scan. Lightweight channelId filter on pre-computed transcript.

Layer 4 — React.memo on transcript items (AgentSessionTranscriptList.tsx)
Spread-copy pattern ensures unchanged items keep their object reference, so memo skips re-render. Only items with new content get new refs.

Correctness safeguards

• Out-of-order event arrivals trigger full transcript rebuild (not incremental)
• 800-event cap trim also triggers rebuild to stay in sync
• latestSessionId propagates even when no transcript items change
• Cross-channel transcript IDs include channelId prefix to prevent collisions
• latestSessionId derived from channel-scoped events (not global)

Notable NITs (not addressed — cosmetic)

• React.memo shallow comparison on TranscriptItemView is intentional: memo saves renders for unchanged items, which is the win

Test plan

• pnpm build passes in desktop/
• Pre-commit hooks pass (biome, formatting)
• Pre-push hooks pass (builds, clippy, tests)
• Manual: open activity sidebar while agent is streaming events, confirm no slowdown after 2+ minutes
• Manual: verify transcript renders correctly with multiple channels active

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