Relay: log upstream events + forward error envelopes to device for diagnostics

Plan 3 smoke surfaced 'session_ended reason=error' on every wake with no
context. The relay now logs every upstream event type at INFO and forwards
any upstream 'error' event to the device as 'error code=upstream_<code>'
so it appears in the Pi's journal alongside the wake/session_ended pair.

Also extends the client's wakeword.predict with per-frame RMS + score
diagnostics and the bridge loop with a 5 s heartbeat for triage.

client/main.py

@@ -43,11 +43,28 @@ def _build_bridge_thread(
     stop_event: threading.Event,
 ) -> threading.Thread:
     def run():
+        frame_counter = 0
+        recent_rms_max = 0.0
         while not stop_event.is_set():
             try:
                 pcm16_bytes = mic_queue.get(timeout=0.25)
             except queue.Empty:
                 continue
+            frame_counter += 1
+            raw = np.frombuffer(pcm16_bytes, dtype=np.int16)
+            rms = float(np.sqrt(np.mean(raw.astype(np.float32) ** 2)))
+            if rms > recent_rms_max:
+                recent_rms_max = rms
+            # Every ~5 s emit a heartbeat so we can see if the InputStream is
+            # actually producing frames and what state we're routing to.
+            if frame_counter % 62 == 0:
+                _log.info(
+                    "bridge frames=%d state=%s wake_en=%s up_en=%s qsize=%d rms_max_5s=%.0f",
+                    frame_counter, sm.state.name,
+                    sm.wakeword_enabled, sm.uplink_enabled, mic_queue.qsize(),
+                    recent_rms_max,
+                )
+                recent_rms_max = 0.0
             if sm.wakeword_enabled:
                 frame = np.frombuffer(pcm16_bytes, dtype=np.int16)
                 if frame.size != FRAME_SAMPLES: