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_registry.py

@@ -0,0 +1,20 @@
+import pytest
+
+from styles import STYLES, get_style
+from styles.blink import BlinkStyle
+from styles.simple import SimpleStyle
+
+
+def test_registry_has_blink_and_simple():
+    assert STYLES["blink"] is BlinkStyle
+    assert STYLES["simple"] is SimpleStyle
+
+
+def test_get_style_returns_class():
+    assert get_style("blink") is BlinkStyle
+    assert get_style("simple") is SimpleStyle
+
+
+def test_get_style_unknown_raises():
+    with pytest.raises(ValueError, match="Unknown alarm style"):
+        get_style("nope")