Directive 3: ESP32 MQTT auto-reconnect, heartbeat, MQTT-state LED, drop logging

- Reconnect and re-subscribe on every (re)connect; log the subscribe result.
- Heartbeat every 5 min on voice/audio_stream (valid silent PCM frame) so the
  Directive-1 watchdog can tell "device dead" from "nobody spoke". voice_bridge
  treats it as silence and ignores it.
- Retained LWT "offline" / "online" presence on voice/status for layer
  attribution in the watchdog.
- LED: green now means MQTT connected AND publishing. Wi-Fi down = red blink,
  Wi-Fi up but MQTT down = amber blink. Never green without MQTT.
- Log the reason on every drop: PubSubClient state code, or the Wi-Fi
  disconnect reason from the Wi-Fi event.
- Bound the I2S read (50 ms) so Wi-Fi/MQTT housekeeping cannot stall; reset a
  stale streaming state on disconnect (the stuck-green bug).
- Document the MQTT/LED contract in README; flag config.h and led_control.h as
  stale drafts so they cannot creep back as a second config.
- Credentials stay placeholders; nothing secret is committed.

Not flashed/tested: no ESP32 and no arduino-cli is reachable from the build
environment. Hardware-in-the-loop verification needs a human at the device.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

led_control.h

@@ -1,4 +1,8 @@
 #pragma once
+// STALE / UNUSED. This header is an early LED draft and is NOT compiled:
+// voice_assistant.ino does not include it. It also defines a conflicting
+// `strip` / `DeviceState` / `currentState`, so including it would break the
+// sketch. LED behaviour now lives in voice_assistant.ino (see renderLed()).
 #include <Freenove_WS2812_Lib_for_ESP32.h>
 #include "config.h"