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/transport/include/mqtt.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <cstdint>
+
 namespace kvida {
 
 // Starts mDNS-based MQTT broker discovery (_mqtt._tcp) and, once found,
@@ -12,4 +14,16 @@ void start_mqtt();
 
 bool mqtt_connected();
 
+// Publishes (retained) Home Assistant MQTT Discovery for a diagnostic
+// "Chip temperature" sensor on first call, then the reading itself.
+// Quick end-to-end proof that a real Sensor implementation's data can
+// reach MQTT/HA -- not the eventual generic publish(topic, value) API.
+void publish_chip_temperature(float celsius);
+
+// Registered by main to receive "set LED color" commands from Home
+// Assistant (an RGB light entity) without transport depending on
+// `drivers` directly.
+using LedColorHandler = void (*)(uint8_t red, uint8_t green, uint8_t blue);
+void set_led_color_handler(LedColorHandler handler);
+
 } // namespace kvida