Fix http_response() truncating envelope via stale _tl_fs_read_len

http_response() builds a JSON envelope wrapping the body. If the caller
previously called fs_read() (which sets _tl_fs_read_len = file_size),
http_worker used that stale value as the response copy length — truncating
the larger envelope to the original file size before it reached
http_send_response. The truncated envelope had the body field cut mid-string;
jp_parse_string_raw failed, env_body = "", and http_send_all sent file_size
bytes of garbage past the empty string.

Fix: reset _tl_fs_read_len = 0 at the start of http_response(). The hint
was set for the raw file bytes; the envelope is a new string and must use
strlen() for its length.

runtime/el_runtime.c

@@ -1961,6 +1961,13 @@ el_val_t http_response(el_val_t status, el_val_t headers_json, el_val_t body) {
     const char* b = EL_CSTR(body);
     if (!b) b = "";
 
+    /* Clear the fs_read binary-length hint: the envelope we're about to build
+     * is a fresh JSON string, not the raw file bytes.  Without this reset,
+     * http_worker would use the stale _tl_fs_read_len (= original file size)
+     * to copy the response — truncating the larger envelope before it reaches
+     * http_send_response and http_parse_envelope.                            */
+    _tl_fs_read_len = 0;
+
     JsonBuf out; jb_init(&out);
     jb_puts(&out, EL_HTTP_RESPONSE_TAG);  /* {"el_http_response":1 */
     jb_puts(&out, ",\"status\":");