refactor: styles own their ringing tone directly (Pattern C)

Drop the injected AlarmSound Protocol and the wecker.Sound class. Each style
now owns its tone by using pygame directly; the runner owns only the audio
engine lifecycle (mixer init/quit), button sampling, and cleanup. This is the
seam for future styles that handle their own tone — a 'talk' style would just
import pygame and play speech, with no shared interface to extend.

- styles/base.py: AlarmStyle.__init__(set_led, music_file) + start() + update()
- styles/blink.py: start() loads+plays music via pygame.mixer.music
- styles/simple.py: owns its beep — synthesises a square-wave buffer in
  module (stdlib array+math) and plays it via pygame.mixer.Sound
- wecker.py: setup() brings up the mixer only; run_alarm constructs the style
  and guards start() with a clean log+exit on failure

Tests: a shared tests/conftest.py stubs RPi.GPIO/pygame in sys.modules before
any SUT import (order-independent, removes duplicated inline mocking); the
wecker mock_pygame fixture patches one fresh pygame mock into wecker + both
style modules so assertions see the same calls.

README: tone-ownership and plugin-contract updated.

styles/blink.py

@@ -1,6 +1,8 @@
 import logging
 import random
 
+import pygame
+
 from styles.base import AlarmStyle
 
 # State Machine states for the puzzle flow.
@@ -21,8 +23,8 @@ class BlinkStyle(AlarmStyle):
     exactly that many times. A wrong count starts a fresh sequence.
     """
 
-    def __init__(self, set_led, sound, music_file=None):
-        super().__init__(set_led, sound, music_file)
+    def __init__(self, set_led, music_file=None):
+        super().__init__(set_led, music_file)
         self.state = STATE_RINGING
         self.target_blinks = 0
         self.user_presses = 0
@@ -35,7 +37,9 @@ class BlinkStyle(AlarmStyle):
     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)
+        pygame.mixer.music.load(self.music_file)
+        pygame.mixer.music.set_volume(1.0)
+        pygame.mixer.music.play(-1)
         self.set_led(False)
 
     def update(self, now, is_pressed):
@@ -130,14 +134,8 @@ class BlinkStyle(AlarmStyle):
 
 if __name__ == "__main__":
     # Self-check: a correct blink sequence stops the alarm.
-    class _StubSound:
-        def play_music(self, path):
-            pass
-
-        def play_beep(self):
-            pass
-
-    style = BlinkStyle(lambda on: None, _StubSound(), "x.mp3")
+    # (Does not call start(), which needs the audio engine.)
+    style = BlinkStyle(lambda on: None, "x.mp3")
 
     assert style.update(100.0, True) is True
     assert style.state == STATE_WAIT_BEFORE_BLINK