Server Sent Events Tests

Test Server-Sent Events (SSE) flows, one-way server-to-client push only (not bidirectional, use websocket-tests for client-to-server messaging): `EventSource` API on the browser side (`onmessage`, `onerror`, `readyState` 0/1/2), event stream format (`data:`, `event:`, `id:`, `retry:`), `Last-Event-ID` reconnect-with-replay header, content-type `text/event-stream`, and HTTP/1.1 connection-pool limits. Use Playwright for browser-side, raw HTTP client for server-side stream tests; k6 load testing (concurrent-stream capacity, connection churn, HTTP/1.1 vs HTTP/2 ceiling) lives in references/sse-load.md. Use when a feature pushes updates over `text/event-stream` and the reconn…