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/base.py

@@ -12,16 +12,19 @@ class AlarmStyle(ABC):
     import-safe in the API process — ``import pygame`` does not initialise
     audio, so the API can import styles just to validate names.
 
-    Lifecycle: construct once with ``set_led``/``music_file``; call ``start()``
-    to begin ringing (kick off the tone and initial LED); then call ``update``
-    every tick. Return ``False`` from ``update`` to stop the alarm.
+    Lifecycle: construct once with ``set_led`` (plus any style-specific
+    keyword config the runner passes); call ``start()`` to begin ringing (kick
+    off the tone and initial LED); then call ``update`` every tick. Return
+    ``False`` from ``update`` to stop the alarm.
+
+    Only ``set_led`` is universal. Style-specific config (e.g. ``music_file``)
+    belongs on the subclass, not here, so the base contract never grows as
+    styles are added; the runner passes config as keyword arguments and each
+    style declares only the ones it uses (extra kwargs are swallowed).
     """
 
-    def __init__(
-        self, set_led: Callable[[bool], None], music_file: str | None = None
-    ):
+    def __init__(self, set_led: Callable[[bool], None], **kwargs):
         self.set_led = set_led
-        self.music_file = music_file
 
     def start(self) -> None:
         """Begin ringing. Override to start the style's tone / initial LED."""