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/el_seed.c

@@ -554,6 +554,94 @@ el_val_t __http_response(el_val_t status, el_val_t headers_json, el_val_t body)
     return http_response(status, headers_json, body);
 }
 
+/* ── HTTP SSE — Server-Sent Events streaming ─────────────────────────────── */
+/*
+ * Thread-local file descriptor stashed by http_worker_v2 before calling the
+ * El handler. El SSE builtins read this to get the raw socket fd.
+ *
+ * Lifecycle:
+ *   http_worker_v2 sets _tl_http_conn_fd = fd  (via el_seed_set_http_conn_fd)
+ *   El handler calls __http_conn_fd()  → receives that fd
+ *   El handler calls __http_sse_open(fd) → sends SSE headers, keeps fd open
+ *   El handler calls __http_sse_send(fd, data) → writes "data: ...\n\n"
+ *   El handler calls __http_sse_close(fd) → closes the fd
+ *   El handler returns "__sse__" sentinel → http_worker_v2 does NOT close fd
+ *
+ * The -1 value means no current connection (guard against misuse outside
+ * a handler context).
+ */
+static __thread int _tl_http_conn_fd = -1;
+
+/* Called by el_runtime.c's http_worker_v2 — not part of the El ABI. */
+void el_seed_set_http_conn_fd(int fd) {
+    _tl_http_conn_fd = fd;
+}
+
+/* __http_conn_fd() — returns the raw fd for the current HTTP connection.
+ * Valid only inside an http_serve_v2 handler before it returns. */
+el_val_t __http_conn_fd(void) {
+    return EL_INT(_tl_http_conn_fd);
+}
+
+/* __http_sse_open(fd) — sends SSE response headers on fd, keeping it open.
+ * Returns 1 on success, 0 on write failure. */
+el_val_t __http_sse_open(el_val_t conn_id) {
+    int fd = (int)(int64_t)conn_id;
+    if (fd < 0) return 0;
+    static const char sse_headers[] =
+        "HTTP/1.1 200 OK\r\n"
+        "Content-Type: text/event-stream\r\n"
+        "Cache-Control: no-cache\r\n"
+        "Connection: keep-alive\r\n"
+        "Access-Control-Allow-Origin: *\r\n"
+        "\r\n";
+    size_t n = sizeof(sse_headers) - 1;  /* exclude NUL */
+    size_t sent = 0;
+    while (sent < n) {
+        ssize_t w = write(fd, sse_headers + sent, n - sent);
+        if (w <= 0) return 0;
+        sent += (size_t)w;
+    }
+    return 1;
+}
+
+/* __http_sse_send(fd, data) — writes one SSE event frame: "data: <data>\n\n".
+ * data must not contain newlines. Returns 1 on success, 0 on client disconnect. */
+el_val_t __http_sse_send(el_val_t conn_id, el_val_t data) {
+    int fd = (int)(int64_t)conn_id;
+    if (fd < 0) return 0;
+    const char* s = EL_CSTR(data);
+    if (!s) s = "";
+    /* Build "data: <s>\n\n" in a single buffer for one write call. */
+    size_t prefix_len = 6;  /* "data: " */
+    size_t slen = strlen(s);
+    size_t total = prefix_len + slen + 2;  /* + "\n\n" */
+    char* buf = malloc(total + 1);
+    if (!buf) return 0;
+    memcpy(buf, "data: ", 6);
+    memcpy(buf + 6, s, slen);
+    buf[6 + slen]     = '\n';
+    buf[6 + slen + 1] = '\n';
+    buf[total] = '\0';
+    size_t sent = 0;
+    int ok = 1;
+    while (sent < total) {
+        ssize_t w = write(fd, buf + sent, total - sent);
+        if (w <= 0) { ok = 0; break; }
+        sent += (size_t)w;
+    }
+    free(buf);
+    return ok ? 1 : 0;
+}
+
+/* __http_sse_close(fd) — closes the SSE connection fd. */
+el_val_t __http_sse_close(el_val_t conn_id) {
+    int fd = (int)(int64_t)conn_id;
+    if (fd < 0) return 0;
+    close(fd);
+    return 1;
+}
+
 /* ── Threading ───────────────────────────────────────────────────────────── */
 /*
  * Design: