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_simple.py

@@ -1,23 +1,49 @@
+from unittest.mock import MagicMock
+
 from styles.simple import SimpleStyle
 
 
+def _make_style():
+    sound = MagicMock()
+    style = SimpleStyle(lambda on: None, sound)
+    return style, sound
+
+
 def test_simple_keeps_ringing_when_not_pressed():
-    calls = []
-    style = SimpleStyle(lambda on: calls.append(on))
+    style, _ = _make_style()
     assert style.update(0.0, False) is True
-    assert calls[-1] is True  # LED solid on while ringing
 
 
 def test_simple_stops_on_first_press():
-    calls = []
-    style = SimpleStyle(lambda on: calls.append(on))
+    style, _ = _make_style()
     assert style.update(0.0, True) is False  # press -> stop
 
 
-def test_simple_led_off_on_release_after_press():
-    # After a press the style stops; verify it never turns the LED off itself
-    # (the runner's cleanup handles that). It only ever asserts LED on.
+def test_simple_led_solid_on():
     calls = []
-    style = SimpleStyle(lambda on: calls.append(on))
-    style.update(0.0, False)
+    sound = MagicMock()
+    style = SimpleStyle(lambda on: calls.append(on), sound)
+    style.start()
     assert calls == [True]
+    style.update(0.0, False)
+    assert calls[-1] is True  # stays on every tick
+
+
+def test_simple_beeps_on_interval():
+    style, sound = _make_style()
+    style.start()
+    style.update(100.0, False)  # first tick -> beep
+    assert sound.play_beep.called
+    sound.play_beep.reset_mock()
+    style.update(100.4, False)  # before interval -> no beep
+    assert not sound.play_beep.called
+    style.update(100.5, False)  # at interval -> beep
+    assert sound.play_beep.called
+
+
+def test_simple_never_plays_music():
+    style, sound = _make_style()
+    style.start()
+    style.update(0.0, False)
+    style.update(0.0, True)  # press -> stop
+    assert not sound.play_music.called