feat: add pluggable alarm styles and migrate wecker.py

Introduce a styles/ plugin package:
- styles/base.py: AlarmStyle ABC (injected set_led, update()->bool contract)
- styles/blink.py: BlinkStyle, the existing count-the-blinks puzzle moved
  out of wecker.py's AlarmClock
- styles/simple.py: SimpleStyle, press-once-to-stop (the new default style)
- styles/__init__.py: STYLES registry + get_style() validator + LEGACY_STYLE

wecker.py now resolves the style via the registry and owns only the shared
music/button/cleanup loop; the --style CLI arg defaults to blink so existing
cron entries keep their behaviour. Unknown styles raise before hardware init.

styles/simple.py

@@ -0,0 +1,18 @@
+from styles.base import AlarmStyle
+
+
+class SimpleStyle(AlarmStyle):
+    """Press the button once to stop the alarm."""
+
+    def update(self, now, is_pressed):
+        # LED solid on so the button is findable in the dark.
+        self.set_led(True)
+        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)
+    assert style.update(0.0, False) is True
+    assert style.update(0.1, True) is False
+    print("simple self-check ok")