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.

styles/simple.py

@@ -1,18 +1,52 @@
 from styles.base import AlarmStyle
 
+# Time between beep retriggers; a short beep followed by this gap gives the
+# classic alarm-clock "beep ... beep ... beep".
+BEEP_INTERVAL = 0.5
+
 
 class SimpleStyle(AlarmStyle):
-    """Press the button once to stop the alarm."""
+    """Press the button once to stop. Rings with a repeating beep.
 
-    def update(self, now, is_pressed):
+    This style plays no music file; it owns its tone via ``sound.play_beep``.
+    """
+
+    def __init__(self, set_led, sound, music_file=None):
+        super().__init__(set_led, sound, music_file)
+        self._next_beep = 0.0
+
+    def start(self):
         # LED solid on so the button is findable in the dark.
         self.set_led(True)
+
+    def update(self, now, is_pressed):
+        self.set_led(True)
+        if now >= self._next_beep:
+            self.sound.play_beep()
+            self._next_beep = now + BEEP_INTERVAL
         return not is_pressed  # first press -> False -> stop
 
 
 if __name__ == "__main__":
-    # Self-check: not pressed keeps ringing; a press stops it.
-    style = SimpleStyle(lambda on: None)
+    # Self-check: not pressed keeps ringing and beeps; a press stops it.
+    class _StubSound:
+        def __init__(self):
+            self.beeps = 0
+
+        def play_music(self, path):
+            raise AssertionError("simple style must never play music")
+
+        def play_beep(self):
+            self.beeps += 1
+
+    sound = _StubSound()
+    style = SimpleStyle(lambda on: None, sound)
+    style.start()
     assert style.update(0.0, False) is True
-    assert style.update(0.1, True) is False
+    assert sound.beeps == 1  # first tick beeps
+    assert style.update(0.4, False) is True  # before interval -> no beep
+    assert sound.beeps == 1
+    assert style.update(0.5, False) is True  # at interval -> beep
+    assert sound.beeps == 2
+    assert style.update(0.6, True) is False  # press -> stop
     print("simple self-check ok")