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_single_instance.py

@@ -1,14 +1,9 @@
 import pytest
-import sys
-from unittest.mock import MagicMock, patch
+from unittest.mock import patch
 
-# Mock hardware modules before importing wecker (tests run off the Pi).
-sys.modules["RPi"] = MagicMock()
-sys.modules["RPi.GPIO"] = MagicMock()
-sys.modules["pygame"] = MagicMock()
-
-import common  # noqa: E402
-import wecker  # noqa: E402
+# Hardware/audio mocks live in tests/conftest.py (shared across all test files).
+import common
+import wecker
 
 
 @pytest.fixture