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/__init__.py

@@ -0,0 +1,23 @@
+from styles.base import AlarmStyle as AlarmStyle
+from styles.blink import BlinkStyle
+from styles.simple import SimpleStyle
+
+# Registry of available alarm styles. Add a line here when you add a style.
+STYLES = {
+    "blink": BlinkStyle,
+    "simple": SimpleStyle,
+}
+
+# Fallback style for legacy cron entries that predate --style, so an upgrade
+# never silently changes an existing alarm's behaviour.
+LEGACY_STYLE = "blink"
+
+
+def get_style(name: str):
+    """Return the style class for ``name`` or raise ValueError."""
+    try:
+        return STYLES[name]
+    except KeyError:
+        raise ValueError(
+            f"Unknown alarm style: {name!r}. Known: {sorted(STYLES)}"
+        )