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.

tests/test_styles_simple.py

@@ -8,6 +8,16 @@ def _make_style():
     return SimpleStyle(lambda on: calls.append(on)), calls
 
 
+def test_simple_ignores_unrelated_config():
+    # SimpleStyle must swallow style-specific kwargs it doesn't use (e.g.
+    # music_file belongs to blink), so the runner can pass config uniformly.
+    style, _ = _make_style()
+    style = SimpleStyle(lambda on: None, music_file="ignored.mp3")
+    with patch("styles.simple.pygame", MagicMock()):
+        style.start()
+        assert style.update(0.0, False) is True
+
+
 def test_simple_keeps_ringing_when_not_pressed():
     style, _ = _make_style()
     with patch("styles.simple.pygame", MagicMock()):