Add Lua runtime: chip temp -> LED color -> HA, verified live

Sandboxed Lua state (espressif/lua v5.5.0, base/table/string/math only,
no io/os/package/debug) exposes a kvida API table (chip_temperature,
set_led, publish) to a compiled-in default script that reads chip
temperature, maps it to an LED color, and publishes the reading to
Home Assistant. Generalizes transport's publish_chip_temperature()
into publish_value(name, value), the first real use of AGENTS.md's
semantic publish API. Requires -DLUA_32BITS globally since this
toolchain's long long support isn't visible to Lua's default build.

Verified on real ESP32-C6 hardware: no crashes/errors across multiple
serial monitor windows, LED changes color, chip_temperature sensor
appears in Home Assistant via MQTT Discovery.

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

components/transport/include/mqtt.h

@@ -7,18 +7,21 @@ namespace kvida {
 // Starts mDNS-based MQTT broker discovery (_mqtt._tcp) and, once found,
 // connects the MQTT client and publishes Home Assistant MQTT Discovery
 // for a diagnostic "connectivity" entity. Safe to call multiple times
-// (no-op if already connecting/connected). No public-facing semantic
-// publish(topic, value) API yet -- deferred until `profiles` has real
-// sensor data to publish (see components/transport/README.md).
+// (no-op if already connecting/connected).
 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);
+// Publishes a named numeric value as a Home Assistant MQTT Discovery
+// sensor: `kvida/<id>/<name>` state topic, `homeassistant/sensor/<id>/<name>/config`
+// discovery topic. This is the semantic publish(name, value) API AGENTS.md
+// describes -- driven by `lua_runtime`'s `kvida.publish()`, not called
+// directly by other components. Discovery is republished (retained, so
+// idempotent) on every call rather than cached behind a per-name
+// "already announced" flag -- simpler, at the cost of a bit of extra
+// retained-message traffic each call. TODO: cache announced names if
+// that traffic becomes a real concern.
+void publish_value(const char *name, double value);
 
 // Registered by main to receive "set LED color" commands from Home
 // Assistant (an RGB light entity) without transport depending on