Busy-ack: client still shows the canned fallback 「看到了，这步做完就接着处理你这条。」 while the host ack is pending #1656

User complaint after #1630 (merged, tip 69a9b2b): the host now composes a context-aware busy-ack (#1629), but while that ack is still pending the client renders the fixed BUSY_ACK_KEY line 「看到了，这步做完就接着处理你这条。」 (core/src/client/busy-ack.js busyAckText fallback). It still reads canned.

Expected: no fixed fallback sentence. While the host ack is pending show only the Bot avatar and a short blinking indicator; when the host's ack arrives, show that text.