refactor: keep style-specific config off the base class via **kwargs

The base AlarmStyle contract carried music_file, forcing SimpleStyle to
accept a parameter it never uses. Move style-specific config to the
subclass: the base __init__ takes only set_led (**kwargs swallows the rest);
each style declares the keyword args it actually uses (BlinkStyle: music_file;
SimpleStyle: none). The runner passes config as keyword args and each style
keeps only what it needs, so the base contract never grows as styles are
added — a future 'talk' style adds talk_file to its own signature, not to the
base. README plugin contract updated.

styles/simple.py

@@ -37,8 +37,8 @@ class SimpleStyle(AlarmStyle):
     wave played through ``pygame.mixer.Sound``.
     """
 
-    def __init__(self, set_led, music_file=None):
-        super().__init__(set_led, music_file)
+    def __init__(self, set_led, **kwargs):
+        super().__init__(set_led, **kwargs)
         self._next_beep = 0.0
         self._beep = None