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.

README.md

@@ -152,7 +152,7 @@ Each alarm has a **style** that decides how you turn it off. Set it per alarm vi
 
 **The style owns its ringing tone.** The runner brings up the audio engine (the pygame mixer) and hands each style the button + LED; the style then produces its own tone directly — `blink` plays a music file, `simple` synthesises a beep, a future `talk` style would play speech. Styles use pygame directly (it's the audio engine, not hardware); the runner still owns mixer init and cleanup. This direct ownership is the seam for future styles that handle their own tone.
 
-Adding a style is a plugin-style drop-in: add a class under `styles/` implementing the `AlarmStyle` contract (`__init__(set_led, music_file)` + `start()` + `update(now, is_pressed) -> bool`) and register one line in `styles/__init__.py`.
+Adding a style is a plugin-style drop-in: add a class under `styles/` implementing the `AlarmStyle` contract (`__init__(set_led, **kwargs)` + `start()` + `update(now, is_pressed) -> bool`) and register one line in `styles/__init__.py`. Only `set_led` is universal; declare any style-specific config as your own keyword args (e.g. `BlinkStyle` takes `music_file`, `SimpleStyle` takes none) — the runner passes config as keyword args and each style keeps only what it uses.
 
 **Backward compatibility:** existing cron entries created before this feature have no `--style` flag and keep running the `blink` puzzle, so an upgrade never silently changes an alarm. To switch an existing alarm to `simple`, re-save it with `setAlarm(id: ..., cronExpression: ..., style: "simple")`.