fix(gateway): harden websocket streaming under client backpressure

docs/INTEGRATION_TESTING.md

@@ -37,7 +37,7 @@ matches use the real 60-second reconnect deadline. It performs these checks:
 2. Verify heartbeat round-trip latency for all six sessions.
 3. Stop consuming one observer stream while Player 1 performs accepted moves;
    intermediate observer updates may be dropped, but player acknowledgements
-   must remain within the reported 250 ms budget.
+   must remain within the reported 500 ms budget.
 4. Flood a second observer with malformed messages. The fixed rate limiter
    closes sustained abuse, and the players remain responsive.
 5. Complete and authoritatively reset two matches through the real disconnect