CollabSocket.send previously called ws.send() whenever the socket was OPEN without checking bufferedAmount, allowing the WebSocket send buffer to grow unbounded under a slow relay or guest. The fix adds 64 KiB high / 32 KiB low backpressure thresholds: new frames are queued when bufferedAmount is above the high watermark, and a 25 ms drain timer retries flushing queued frames once the buffer drops below the low watermark, preserving send order via the existing #sendChain. Overflow still drops frames once #pendingSends reaches MAX_PENDING_SENDS, and the drain timer is cleared on close, reconnect, and fatal failure. Verified with `bun test packages/coding-agent/test/collab` (64 pass, 0 fail).
Closes#4248
can1357