feat(gateway): butler filler line while Tatlock works

Tatlock turns take 10-25s, which is a long silence after a request. Speak
a canned "Let me check on that for you, sir" immediately (synthesized once
and cached), then re-assert the thinking state so the device keeps its
effort-face spinner up until the real reply arrives. On the device, guard
the playback-done handler so the filler audio finishing doesn't drop the
spinner back to idle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LKPbR6DY2JygHbyLjxm7Uu

gateway/src/desklock_gateway/config.py

@@ -19,6 +19,9 @@ class Settings(BaseSettings):
     tts_model: str = "speaches-ai/Kokoro-82M-v1.0-ONNX"
     tts_voice: str = "bm_george"
 
+    # Spoken immediately after a query is accepted, to fill the long Tatlock wait.
+    filler_text: str = "Let me check on that for you, sir."
+
     # embedded fallback only (requires the [speech] extra)
     embedded_stt_model: str = "small"
     embedded_stt_device: str = "cuda"

gateway/src/desklock_gateway/main.py

@@ -24,6 +24,21 @@ PCM_CHUNK_BYTES = 4096
 # device registry: every endpoint that ever connected, keyed by client IP
 DEVICES: dict[str, dict] = {}
 
+# Butler filler audio (the "let me check…" line) is synthesized once, then replayed.
+_filler_pcm: bytes | None = None
+_filler_tried = False
+
+
+async def _ensure_filler() -> bytes | None:
+    global _filler_pcm, _filler_tried
+    if not _filler_tried:
+        _filler_tried = True
+        try:
+            _filler_pcm = await asyncio.to_thread(tts.synthesize, settings.filler_text)
+        except Exception:
+            logger.exception("filler synth failed; continuing without it")
+    return _filler_pcm
+
 
 def _now() -> str:
     return datetime.now(timezone.utc).astimezone().isoformat(timespec="seconds")
@@ -111,6 +126,17 @@ async def _handle_utterance(ws: WebSocket, tatlock: TatlockClient, pcm: bytes) -
         await ws.send_json({"type": "state", "value": "idle"})
         return
 
+    # Acknowledge immediately with a canned line so the long Tatlock wait isn't dead
+    # air, then re-assert "thinking" to keep the spinner up while it works.
+    filler = await _ensure_filler()
+    if filler:
+        await ws.send_json({"type": "reply_text", "text": settings.filler_text})
+        await ws.send_json({"type": "audio_start", "sample_rate": settings.sample_rate})
+        for offset in range(0, len(filler), PCM_CHUNK_BYTES):
+            await ws.send_bytes(filler[offset : offset + PCM_CHUNK_BYTES])
+        await ws.send_json({"type": "audio_end"})
+        await ws.send_json({"type": "state", "value": "thinking"})
+
     reply = await tatlock.ask(transcript)
     await ws.send_json({"type": "reply_text", "text": reply})