fix: suppress stale WebSocket close message after stopping waterfall

stop() sets _ws = null before the async onclose fires, so the handler
now early-returns when _ws is null instead of showing the misleading
"WebSocket closed before ready" retry message.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/js/modes/waterfall.js

@@ -2996,6 +2996,8 @@ const Waterfall = (function () {
         };
 
         _ws.onclose = () => {
+            // stop() sets _ws = null before the async onclose fires.
+            if (!_ws) return;
             if (!_wsOpened && _active) {
                 // Wait for timeout-based fallback; avoid flapping to SSE on brief close/retry.
                 _setStatus('WebSocket closed before ready. Waiting to retry/fallback...');