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/blink.py

@@ -21,8 +21,8 @@ class BlinkStyle(AlarmStyle):
     exactly that many times. A wrong count starts a fresh sequence.
     """
 
-    def __init__(self, set_led):
-        super().__init__(set_led)
+    def __init__(self, set_led, sound, music_file=None):
+        super().__init__(set_led, sound, music_file)
         self.state = STATE_RINGING
         self.target_blinks = 0
         self.user_presses = 0
@@ -32,6 +32,12 @@ class BlinkStyle(AlarmStyle):
         self._blink_phases = 0
         self._blink_next_toggle: float | None = None
 
+    def start(self):
+        # The puzzle rings with music on an endless loop; the LED stays off
+        # until the button is pressed to start the blink sequence.
+        self.sound.play_music(self.music_file)
+        self.set_led(False)
+
     def update(self, now, is_pressed):
         button_just_pressed = False
         if is_pressed and not self.button_was_pressed:
@@ -124,7 +130,14 @@ class BlinkStyle(AlarmStyle):
 
 if __name__ == "__main__":
     # Self-check: a correct blink sequence stops the alarm.
-    style = BlinkStyle(lambda on: None)
+    class _StubSound:
+        def play_music(self, path):
+            pass
+
+        def play_beep(self):
+            pass
+
+    style = BlinkStyle(lambda on: None, _StubSound(), "x.mp3")
 
     assert style.update(100.0, True) is True
     assert style.state == STATE_WAIT_BEFORE_BLINK