fix: eliminate LED flicker during knob turns

Two targeted fixes:

1. Deduplicate LED writes in handle_knob: compute new colours and only
   call send_leds() when they differ from last_led_colors.  A single
   physical knob turn produces 20-50 ADC samples in quick succession;
   without this guard each sample triggers a 47-byte write and the
   firmware can't keep up, causing visible flicker.

2. Gate reapply_app_volumes on a 200 ms knob-quiet period.  Calling
   playerctl/pulsectl while the user is actively turning a knob stalls
   the main loop, backing up serial data, missing heartbeats, and
   causing the device LEDs to time out mid-turn.

Both state variables (last_led_colors, last_knob_event) are mutable
lists initialised in main() and threaded through handle_knob, keeping
module-level globals out of the picture.

Adds 6 new unit tests (56 total, all passing).

tests/test_protocol.py

@@ -4,8 +4,17 @@ No external dependencies — these are pure-function tests.
 """
 
 import pytest
+from unittest.mock import MagicMock, call, patch
 
-from turnup.turnupd import knob_to_norm, knob_to_volume, parse_messages, KNOB_MAX, VOLUME_MAX
+from turnup.turnupd import (
+    KNOB_MAX,
+    NUM_KNOBS,
+    VOLUME_MAX,
+    handle_knob,
+    knob_to_norm,
+    knob_to_volume,
+    parse_messages,
+)
 
 
 # ── knob_to_norm ──────────────────────────────────────────────────────────────
@@ -126,3 +135,81 @@ class TestParseMessages:
         msgs, remainder = parse_messages(bytearray())
         assert msgs == []
         assert remainder == bytearray()
+
+
+# ── handle_knob ───────────────────────────────────────────────────────────────
+
+def _make_knob_fixtures(knob_id: int = 0, action: str = "sink_volume"):
+    """Return (config, pulse_mock, ser_mock, knob_norms, last_led_colors, last_knob_event)."""
+    config = {
+        "knobs": {
+            str(knob_id): {
+                "action": action,
+                "target": "default",
+                "led": {"low_color": "#000000", "high_color": "#ffffff"},
+            }
+        }
+    }
+    pulse = MagicMock()
+    ser   = MagicMock()
+    knob_norms       = [0.0] * NUM_KNOBS
+    last_led_colors  = [(0, 0, 0)] * NUM_KNOBS
+    last_knob_event  = [0.0]
+    return config, pulse, ser, knob_norms, last_led_colors, last_knob_event
+
+
+class TestHandleKnobLEDDedup:
+    """Fix 1 — duplicate LED packets must be suppressed."""
+
+    def test_sends_leds_when_color_changes(self):
+        config, pulse, ser, knob_norms, llc, lke = _make_knob_fixtures()
+        # Turn knob 0 to max; color should change from (0,0,0).
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        assert ser.write.called
+
+    def test_skips_leds_when_color_unchanged(self):
+        config, pulse, ser, knob_norms, llc, lke = _make_knob_fixtures()
+        # First call sets the color.
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        write_count_after_first = ser.write.call_count
+        # Second call with same value — color is identical, no new write.
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        assert ser.write.call_count == write_count_after_first
+
+    def test_last_led_colors_updated_after_send(self):
+        config, pulse, ser, knob_norms, llc, lke = _make_knob_fixtures()
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        # last_led_colors must not remain all-zero after a successful send.
+        assert llc != [(0, 0, 0)] * NUM_KNOBS
+
+    def test_color_change_after_stable_triggers_send(self):
+        config, pulse, ser, knob_norms, llc, lke = _make_knob_fixtures()
+        # Reach a stable state at max value.
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        count_stable = ser.write.call_count
+        # Now change to zero — should trigger another send.
+        handle_knob(0, 0, config, pulse, ser, knob_norms, llc, lke)
+        assert ser.write.call_count == count_stable + 1
+
+
+class TestHandleKnobLastKnobEvent:
+    """Fix 2 — last_knob_event[0] must be updated on every call."""
+
+    def test_last_knob_event_updated(self):
+        import time
+        config, pulse, ser, knob_norms, llc, lke = _make_knob_fixtures()
+        before = time.monotonic()
+        handle_knob(0, 500, config, pulse, ser, knob_norms, llc, lke)
+        after = time.monotonic()
+        assert before <= lke[0] <= after
+
+    def test_last_knob_event_updated_even_when_led_unchanged(self):
+        import time
+        config, pulse, ser, knob_norms, llc, lke = _make_knob_fixtures()
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        first_ts = lke[0]
+        import time as _t; _t.sleep(0.01)
+        handle_knob(0, KNOB_MAX, config, pulse, ser, knob_norms, llc, lke)
+        # Timestamp must advance even though color did not change.
+        assert lke[0] > first_ts