Add chip temperature sensor and RGB LED control, verified live in HA

Quick end-to-end data-flow proof using only what's on the ESP32-C6-
DevKitC-1 itself (no sensors on the bare devkit -- confirmed via
research: BOOT button, WS2812 LED, no external sensors):

- drivers::chip_temperature_* wraps ESP-IDF's built-in
  temperature_sensor driver (SoC die temperature).
- drivers::rgb_led_* wraps the onboard WS2812 (GPIO8) via the
  espressif/led_strip managed component (another external registry
  dependency, like mdns/mqtt).
- profiles::ChipTemperatureSensor is the first real implementation of
  the Sensor interface designed earlier, converting the driver's float
  reading to the fixed-point convention once at the boundary.
- transport/mqtt.cpp publishes chip temperature as an HA "temperature"
  sensor every 30s, and exposes the LED as an HA "light" entity
  (rgb_command_topic) -- the first use of MQTT subscribe in this
  project, not just publish.

Added a ChipTemperature SensorChannel (distinct from
AmbientTemperature -- different meaning/range) to sensor.h.

Verified live: both the chip temperature reading and LED color control
work from Home Assistant against the real device.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

components/drivers/include/chip_temperature.h

@@ -0,0 +1,11 @@
+#pragma once
+
+namespace kvida::drivers {
+
+// Wraps ESP-IDF's temperature_sensor driver (die temperature, not
+// ambient -- there is no external temperature sensor on the ESP32-C6
+// DevKitC). Call init() once before read_celsius().
+void chip_temperature_init();
+float chip_temperature_read_celsius();
+
+} // namespace kvida::drivers

components/drivers/include/rgb_led.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <cstdint>
+
+namespace kvida::drivers {
+
+// Wraps the WS2812 addressable RGB LED on the ESP32-C6-DevKitC-1 (GPIO8).
+void rgb_led_init();
+void rgb_led_set_color(uint8_t red, uint8_t green, uint8_t blue);
+
+} // namespace kvida::drivers