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/profiles/include/chip_temperature_sensor.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "sensor.h"
+
+namespace kvida {
+
+// First real implementation of the Sensor interface (see
+// components/sensors/include/sensor.h) -- wraps the ESP32-C6's internal
+// die temperature sensor (kvida::drivers::chip_temperature_*).
+class ChipTemperatureSensor : public Sensor {
+public:
+    const char *id() const override { return "chip_temperature"; }
+    void begin() override;
+
+    bool supports(SensorChannel channel) const override
+    {
+        return channel == SensorChannel::ChipTemperature;
+    }
+
+    bool fetch() override;
+    bool get(SensorChannel channel, SensorValue &out) override;
+
+private:
+    bool fetched_ = false;
+    SensorValue last_{};
+};
+
+} // namespace kvida