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.

tests/test_styles_simple.py

@@ -1,49 +1,55 @@
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 from styles.simple import SimpleStyle
 
 
 def _make_style():
-    sound = MagicMock()
-    style = SimpleStyle(lambda on: None, sound)
-    return style, sound
+    calls = []
+    return SimpleStyle(lambda on: calls.append(on)), calls
 
 
 def test_simple_keeps_ringing_when_not_pressed():
     style, _ = _make_style()
-    assert style.update(0.0, False) is True
+    with patch("styles.simple.pygame", MagicMock()):
+        style.start()
+        assert style.update(0.0, False) is True
 
 
 def test_simple_stops_on_first_press():
     style, _ = _make_style()
-    assert style.update(0.0, True) is False  # press -> stop
+    with patch("styles.simple.pygame", MagicMock()):
+        style.start()
+        assert style.update(0.0, True) is False  # press -> stop
 
 
 def test_simple_led_solid_on():
-    calls = []
-    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
+    style, calls = _make_style()
+    with patch("styles.simple.pygame", MagicMock()):
+        style.start()
+        style.update(0.0, False)
+    assert calls == [True, True]  # start sets LED on; update keeps it on
 
 
 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
+    style, _ = _make_style()
+    with patch("styles.simple.pygame", MagicMock()) as pg:
+        style.start()
+        style.update(100.0, False)  # first tick -> beep
+        assert pg.mixer.Sound.return_value.play.called
+        pg.mixer.Sound.return_value.play.reset_mock()
+
+        style.update(100.4, False)  # before interval -> no beep
+        assert not pg.mixer.Sound.return_value.play.called
+
+        style.update(100.5, False)  # at interval -> beep
+        assert pg.mixer.Sound.return_value.play.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
+    style, _ = _make_style()
+    with patch("styles.simple.pygame", MagicMock()) as pg:
+        style.start()
+        style.update(0.0, False)
+        style.update(0.0, True)  # press -> stop
+    assert not pg.mixer.music.play.called
+    assert not pg.mixer.music.load.called