5.12 — Chat SSE event types (mock, GREEN)

Phase: 5 — Chat + Rate Limiter (Surface D) Stage: GREEN

Chat SSE events: {"type":"queue"|"reasoning"|"content"|"done"} implemented; "error" not yet emitted (mock never errors)

Done — app/api/chat/route.ts. Frontend should render reasoning in a collapsible block (this is the demo).