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>

src/planemapper/gpio_ctrl.py

@@ -1,11 +1,29 @@
+import logging
+
+from gpiozero import LED, Button
+
+from planemapper.constants import RESET_HOLD_S
+
+log = logging.getLogger(__name__)
+
+BUTTON_GPIO_PIN = 17
+LED_GPIO_PIN = 27
+
+
 class ButtonHoldDetector:
+    def __init__(self, pin: int = BUTTON_GPIO_PIN) -> None:
+        self._button = Button(pin, hold_time=RESET_HOLD_S)
+
     def check(self) -> bool:
-        return False
+        return self._button.is_held
 
 
 class LEDController:
+    def __init__(self, pin: int = LED_GPIO_PIN) -> None:
+        self._led = LED(pin)
+
     def on(self) -> None:
-        pass
+        self._led.on()
 
     def off(self) -> None:
-        pass
+        self._led.off()