Add temporary diagnostics for LED flicker investigation

Logs (WARNING level) when: multiple heartbeats arrive in one serial read
batch, a heartbeat gap exceeds 0.5s, or reapply_app_volumes takes >0.15s.
Suspected cause: synchronous playerctl/pulsectl calls in the reapply path
occasionally stalling the main loop long enough for backed-up heartbeats
to flush in a burst, each triggering an LED resend. To be removed once
confirmed/fixed.

src/turnup/turnupd.py

@@ -421,11 +421,20 @@ def main() -> None:
                 send_leds(ser, initial_colors)
                 last_led_colors[:] = initial_colors
 
+                heartbeat_count = 0
+                last_heartbeat_ts = 0.0
+
                 while True:
                     data = ser.read(64)
                     if data:
                         buf.extend(data)
                         messages, buf = parse_messages(buf)
+                        hb_in_batch = sum(1 for m in messages if m["type"] == "heartbeat")
+                        if hb_in_batch > 1:
+                            log.warning(
+                                "DIAG: %d heartbeats in one read batch (backlog flush)",
+                                hb_in_batch,
+                            )
                         for msg in messages:
                             if msg["type"] == "knob":
                                 handle_knob(
@@ -439,6 +448,14 @@ def main() -> None:
                                     msg["id"], msg["action"], config, pulse, active_window
                                 )
                             elif msg["type"] == "heartbeat":
+                                now_hb = time.monotonic()
+                                heartbeat_count += 1
+                                if last_heartbeat_ts and (now_hb - last_heartbeat_ts) > 0.5:
+                                    log.warning(
+                                        "DIAG: heartbeat gap %.2fs before heartbeat #%d",
+                                        now_hb - last_heartbeat_ts, heartbeat_count,
+                                    )
+                                last_heartbeat_ts = now_hb
                                 new_colors = all_led_colors(config, knob_norms)
                                 send_leds(ser, new_colors)
                                 last_led_colors[:] = new_colors
@@ -467,7 +484,11 @@ def main() -> None:
                     knob_quiet = now - last_knob_event[0] >= 0.2
                     if (pulse.drain_events() or now - last_reapply >= 1.0) and knob_quiet:
                         last_reapply = now
+                        _t0 = time.monotonic()
                         reapply_app_volumes(config, pulse, knob_norms)
+                        _elapsed = time.monotonic() - _t0
+                        if _elapsed > 0.15:
+                            log.warning("DIAG: reapply_app_volumes took %.3fs", _elapsed)
 
         except serial.SerialException as exc:
             log.warning("Serial error: %s — retrying in 3 s", exc)