refactor(sse): serialize backend writes and add directory-aware frontend batching

[chriswritescode-dev]

SSE writes from the backend could arrive out of order under concurrent load. The frontend parts batcher also lacked directory awareness, making it unreliable when multiple directories are monitored.

Changes:

• Add QueuedSSEWriter that serializes SSE frames via promise chaining with backpressure handling and error isolation
• Refactor SSE route to use QueuedSSEWriter instead of raw encoder
• Make frontend parts batcher directory-aware: operations grouped by (sessionID, directory) composite key; flush() accepts optional target filter
• Decouple batcher lifecycle from directory dependency in useSSE hook
• Add comprehensive tests for all new behavior

Files

• backend/src/routes/sse-writer.ts
• backend/src/routes/sse.ts
• backend/test/routes/sse-writer.test.ts
• frontend/src/hooks/useSSE.ts
• frontend/src/hooks/useSSE.test.tsx
• frontend/src/lib/partsBatcher.ts
• frontend/src/lib/partsBatcher.test.ts
• frontend/src/lib/opencode-event-stream/openCodeEventStream.ts
• frontend/src/lib/opencode-event-stream/testTransport.ts
• frontend/src/lib/opencode-event-stream/tests/openCodeEventStream.test.ts