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.

tests/test_styles_simple.py

@@ -0,0 +1,23 @@
+from styles.simple import SimpleStyle
+
+
+def test_simple_keeps_ringing_when_not_pressed():
+    calls = []
+    style = SimpleStyle(lambda on: calls.append(on))
+    assert style.update(0.0, False) is True
+    assert calls[-1] is True  # LED solid on while ringing
+
+
+def test_simple_stops_on_first_press():
+    calls = []
+    style = SimpleStyle(lambda on: calls.append(on))
+    assert style.update(0.0, True) is False  # press -> stop
+
+
+def test_simple_led_off_on_release_after_press():
+    # After a press the style stops; verify it never turns the LED off itself
+    # (the runner's cleanup handles that). It only ever asserts LED on.
+    calls = []
+    style = SimpleStyle(lambda on: calls.append(on))
+    style.update(0.0, False)
+    assert calls == [True]