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()

el-compiler/runtime/legacy/el_runtime.h

@@ -176,6 +176,11 @@ void      http_set_handler_v2(el_val_t name);
  * auto-content-type contract for legacy handlers that return plain bodies. */
 el_val_t  http_response(el_val_t status, el_val_t headers_json, el_val_t body);
 
+/* SSE connection fd — set by http_worker_v2 before calling the El handler,
+ * cleared afterwards. Defined in el_seed.c; called from el_runtime.c.
+ * The getter is exposed as __http_conn_fd() to El programs. */
+void      el_seed_set_http_conn_fd(int fd);
+
 /* HTTP timeout — every libcurl request honors EL_HTTP_TIMEOUT_MS (default
  * 60000ms). Read lazily on first use, so setting the env var any time before
  * the first http_* call is sufficient.                                    */