Implement GPIO button hold detection and LED feedback (story 4-1)

Replace stub ButtonHoldDetector and LEDController with real gpiozero
implementations; update tests to use MockFactory so they run without
physical GPIO hardware.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_gpio_ctrl.py

@@ -1,12 +1,26 @@
+import pytest
+from gpiozero import Device
+from gpiozero.pins.mock import MockFactory
+
 from planemapper.gpio_ctrl import ButtonHoldDetector, LEDController
 
 
+@pytest.fixture(autouse=True)
+def mock_gpio() -> None:
+    Device.pin_factory = MockFactory()
+
+
 def test_button_hold_detector_returns_bool() -> None:
     detector = ButtonHoldDetector()
     result = detector.check()
     assert isinstance(result, bool)
 
 
+def test_button_hold_detector_not_held_by_default() -> None:
+    detector = ButtonHoldDetector()
+    assert detector.check() is False
+
+
 def test_led_controller_on_off_no_exception() -> None:
     led = LEDController()
     led.on()