runtime: native SSE streaming — http_sse_open/send/close

Add Server-Sent Events support to the El runtime. El v2 handlers can now
hold HTTP connections open and push events in real time.

New builtins in el_seed.c:
  __http_conn_fd()          — retrieve raw fd from thread-local set by worker
  __http_sse_open(fd)       — send SSE headers (text/event-stream), keep-alive
  __http_sse_send(fd, data) — write "data: <data>\n\n" frame
  __http_sse_close(fd)      — close the connection fd

http_worker_v2 in legacy/el_runtime.c now:
  - stashes the fd via el_seed_set_http_conn_fd() before calling the handler
  - detects the "__sse__" sentinel return value to skip http_send_response
    and skip close(fd) — SSE handler took ownership of the fd
  - clears the thread-local after the handler returns

El wrappers added to runtime/http.el:
  http_conn_fd() http_sse_open(fd) http_sse_send(fd, data)
  http_sse_close(fd) http_sse_sentinel()

runtime/http.el

@@ -169,3 +169,53 @@ fn http_serve_v2(port: Int, handler: String) {
 fn http_response(status: Int, headers_json: String, body: String) -> String {
     return __http_response(status, headers_json, body)
 }
+
+// ── SSE — Server-Sent Events streaming ───────────────────────────────────────
+//
+// Usage pattern for an SSE handler:
+//
+//   fn my_handler(method: String, path: String, headers: Map<String, String>, body: String) -> String {
+//       let fd: Int = http_conn_fd()
+//       http_sse_open(fd)
+//       http_sse_send(fd, "hello")
+//       http_sse_send(fd, "world")
+//       http_sse_close(fd)
+//       return http_sse_sentinel()
+//   }
+//
+// The sentinel return value tells http_serve_v2 NOT to close the connection
+// automatically — the handler already closed it via http_sse_close.
+
+// http_conn_fd returns the raw file descriptor for the current HTTP connection.
+// Only valid inside an http_serve_v2 handler, before the handler returns.
+// Use with http_sse_open / http_sse_send / http_sse_close for streaming.
+fn http_conn_fd() -> Int {
+    return __http_conn_fd()
+}
+
+// http_sse_open sends SSE response headers on the current connection,
+// keeping it open for streaming. Call once at the start of an SSE handler.
+// Returns true on success.
+fn http_sse_open(fd: Int) -> Bool {
+    return __http_sse_open(fd)
+}
+
+// http_sse_send writes one SSE event to the connection.
+// data should not contain newlines (they are added automatically).
+// Returns true if the write succeeded (client still connected).
+fn http_sse_send(fd: Int, data: String) -> Bool {
+    return __http_sse_send(fd, data)
+}
+
+// http_sse_close closes the SSE connection.
+fn http_sse_close(fd: Int) {
+    __http_sse_close(fd)
+    return
+}
+
+// http_sse_sentinel is the return value an SSE handler must return
+// to tell the HTTP server NOT to close the connection automatically.
+// The handler takes ownership of the fd and closes it via http_sse_close.
+fn http_sse_sentinel() -> String {
+    return "__sse__"
+}