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.

tests/test_styles_blink.py

@@ -0,0 +1,97 @@
+from styles.blink import (
+    STATE_BLINKING,
+    STATE_EVALUATING,
+    STATE_WAIT_BEFORE_BLINK,
+    STATE_WAIT_BEFORE_RETRY,
+    STATE_WAIT_FOR_INPUT,
+    BlinkStyle,
+)
+
+
+def _led():
+    """Return (calls list, set_led callable recording each call)."""
+    calls = []
+    return calls, lambda on: calls.append(on)
+
+
+def test_press_starts_puzzle():
+    _, set_led = _led()
+    style = BlinkStyle(set_led)
+    assert style.update(100.0, True) is True
+    assert style.state == STATE_WAIT_BEFORE_BLINK
+
+
+def test_blink_correct_sequence_stops(monkeypatch):
+    _, set_led = _led()
+    style = BlinkStyle(set_led)
+
+    style.update(100.0, True)  # press -> start puzzle
+    assert style.state == STATE_WAIT_BEFORE_BLINK
+
+    monkeypatch.setattr("styles.blink.random.randint", lambda a, b: 3)
+    style.update(103.1, False)  # 3s passed -> blinking, 3 blinks
+    assert style.state == STATE_BLINKING
+    assert style.target_blinks == 3
+
+    style.update(105.0, False)  # run through the blink sequence
+    assert style.state == STATE_WAIT_FOR_INPUT
+
+    # Press exactly 3 times
+    for t in (105.5, 106.0, 106.5):
+        style.update(t, True)
+        style.update(t + 0.05, False)
+    assert style.user_presses == 3
+
+    style.update(109.7, False)  # 3s idle -> evaluating
+    assert style.state == STATE_EVALUATING
+    assert style.update(109.8, False) is False  # correct -> stop
+
+
+def test_blink_incorrect_retries():
+    _, set_led = _led()
+    style = BlinkStyle(set_led)
+    style.state = STATE_WAIT_FOR_INPUT
+    style.target_blinks = 3
+
+    style.update(100.0, True)  # one press
+    style.update(100.1, False)
+
+    style.last_interaction_time = 100.1
+    style.update(103.2, False)  # -> evaluating
+    keep_running = style.update(103.3, False)  # 1 != 3 -> retry
+
+    assert keep_running is True
+    assert style.state == STATE_WAIT_BEFORE_RETRY
+
+
+def test_blinking_non_blocking():
+    calls, set_led = _led()
+    style = BlinkStyle(set_led)
+    style.state = STATE_BLINKING
+    style.target_blinks = 2
+
+    style.update(100.0, False)  # start of blinking: LED on
+    assert style.state == STATE_BLINKING
+    assert calls[-1] is True
+
+    style.update(100.3, False)  # mid-blink toggles by elapsed time
+    assert calls[-1] is False
+
+    # 2 blinks * 2 phases * 0.3s = 1.2s
+    style.update(101.2, False)
+    assert style.state == STATE_WAIT_FOR_INPUT
+    assert calls[-1] is False
+
+
+def test_blinking_advances_time():
+    _, set_led = _led()
+    style = BlinkStyle(set_led)
+    style.state = STATE_BLINKING
+    style.target_blinks = 4
+
+    # 4 blinks * 2 phases * 0.3s = 2.4s
+    style.update(100.0, False)
+    style.update(102.4, False)
+
+    assert style.state == STATE_WAIT_FOR_INPUT
+    assert style.last_interaction_time == 102.4