:sig
pid_t server_start_http(String key, String socket_fn_or_port, String call_uce_filename, String call_function = "")

:params
key : runtime-wide server key
socket_fn_or_port : TCP port number or Unix socket path to listen on
call_uce_filename : `.uce` file containing `SERVE_HTTP` handlers
call_function : optional named `SERVE_HTTP:name` handler
return value : listener process PID, or `0` when it could not be started

:see
>sys
server_stop
task

:content
Starts or updates a custom HTTP listener backed by a UCE handler file.

The server key is runtime-wide. Calling `server_start_http()` again with the same key and the same bind target updates the mapped UCE file/function without restarting the listener. If the bind target changes, UCE stops the old listener and starts a replacement.

`socket_fn_or_port` may be a numeric TCP port such as `"19091"` or a Unix socket path.

Handler files use `SERVE_HTTP`:

```uce
SERVE_HTTP(Request* req)
{
    req->header["Content-Type"] = "text/plain; charset=utf-8";
    print("hello from custom HTTP\n");
}

SERVE_HTTP:admin(Request* req)
{
    print("named handler\n");
}
```

The custom server dispatcher uses the same nonblocking HTTP listener machinery as the built-in HTTP/WebSocket path, and hands request/response data through the normal `Request` object. Handler code may block in the first implementation; future worker dispatch can be added without changing this API.
