feat: styles own their ringing tone (simple beeps, blink plays music)

Move audio ownership out of the shared runner and into each style via an
injected Sound capability (play_music(path) / play_beep()). The runner now
only owns button sampling and cleanup; the style kicks off its own tone in
start() and drives the LED.

- simple: a repeating square-wave beep (ordinary alarm-clock tone), no music
  file required. The beep is synthesised in memory as signed-16-bit stereo
  PCM (stdlib array+math) and played via pygame.mixer.Sound — no shipped
  audio asset, no new dependency.
- blink: unchanged behaviour — music on an endless loop from --music-file /
  MUSIC_FILE / the default track, via pygame.mixer.music.
- AlarmStyle contract gains sound + music_file in __init__ and a start()
  lifecycle hook; AlarmSound Protocol documents the audio seam for future
  styles that handle their own tone.
- setup() no longer loads music (that is the style's job now).

README updated: per-style ringing tone, the --music-file note (blink only),
and the extended plugin contract.

tests/test_styles_blink.py

@@ -1,3 +1,5 @@
+from unittest.mock import MagicMock
+
 from styles.blink import (
     STATE_BLINKING,
     STATE_EVALUATING,
@@ -8,23 +10,31 @@ from styles.blink import (
 )
 
 
-def _led():
-    """Return (calls list, set_led callable recording each call)."""
+def _make_blink(music_file="music.mp3"):
+    """Return (style, sound_mock) for a BlinkStyle ready to start."""
+    sound = MagicMock()
+    style = BlinkStyle(lambda on: None, sound, music_file)
+    return style, sound
+
+
+def test_start_plays_music_and_led_off():
     calls = []
-    return calls, lambda on: calls.append(on)
+    sound = MagicMock()
+    style = BlinkStyle(lambda on: calls.append(on), sound, "track.mp3")
+    style.start()
+    sound.play_music.assert_called_once_with("track.mp3")
+    assert calls == [False]  # LED off while music plays until a press starts the puzzle
+    assert style.state == 0  # STATE_RINGING until a press
 
 
 def test_press_starts_puzzle():
-    _, set_led = _led()
-    style = BlinkStyle(set_led)
+    style, _ = _make_blink()
     assert style.update(100.0, True) is True
     assert style.state == STATE_WAIT_BEFORE_BLINK
 
 
 def test_blink_correct_sequence_stops(monkeypatch):
-    _, set_led = _led()
-    style = BlinkStyle(set_led)
-
+    style, _ = _make_blink()
     style.update(100.0, True)  # press -> start puzzle
     assert style.state == STATE_WAIT_BEFORE_BLINK
 
@@ -48,8 +58,7 @@ def test_blink_correct_sequence_stops(monkeypatch):
 
 
 def test_blink_incorrect_retries():
-    _, set_led = _led()
-    style = BlinkStyle(set_led)
+    style, _ = _make_blink()
     style.state = STATE_WAIT_FOR_INPUT
     style.target_blinks = 3
 
@@ -65,8 +74,9 @@ def test_blink_incorrect_retries():
 
 
 def test_blinking_non_blocking():
-    calls, set_led = _led()
-    style = BlinkStyle(set_led)
+    calls = []
+    style, _ = _make_blink()
+    style.set_led = calls.append
     style.state = STATE_BLINKING
     style.target_blinks = 2
 
@@ -84,8 +94,7 @@ def test_blinking_non_blocking():
 
 
 def test_blinking_advances_time():
-    _, set_led = _led()
-    style = BlinkStyle(set_led)
+    style, _ = _make_blink()
     style.state = STATE_BLINKING
     style.target_blinks = 4