firmware: retarget camera-node MQTT bridge to XIAO ESP32-C6

The MCU changed from ESP32 Dev Board to a Seeed Studio XIAO ESP32-C6,
but the firmware still targeted esp32dev. Retarget it and fix the
build so it compiles and flashes for the C6.

platformio.ini:
- env esp32-mqtt -> seeed_xiao_esp32c6 on the pioarduino platform fork
  (mainline espressif32 lags the Arduino-core 3.x the C6 needs)
- add ARDUINO_USB_MODE=1 / ARDUINO_USB_CDC_ON_BOOT=1 for Serial over
  the C6 native USB
- fix build_src_filter ordering in BOTH envs: -<*.cpp> ran last and
  re-excluded the target, leaving setup()/loop() undefined at link

esp32-mqtt-bridge.cpp:
- UART Serial2 RX16/TX17 -> Serial1 RX=D7/TX=D6 (XIAO C6 mapping)
- status LED GPIO2 -> D1 (green channel of the RGB STAT LED)
- fix pre-existing ArduinoJson v7 / PubSubClient build errors
  (.c_str() on a const char*, String topic where const char* required)

Verified: builds clean and boots on hardware (native-USB serial banner
confirmed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/platformio.ini

@@ -9,16 +9,16 @@
 ;   (TX/RX)           (RX16/TX17)
 ;
 ; Build:
-;   pio run -e esp8266-camera    (ESP8266 D1 Mini — camera bridge)
-;   pio run -e esp32-mqtt        (ESP32 Dev Board — MQTT bridge)
+;   pio run -e esp8266-camera        (ESP8266 — GoPro camera bridge)
+;   pio run -e seeed_xiao_esp32c6    (XIAO ESP32-C6 — MQTT bridge)
 ;
 ; Upload:
 ;   pio run -e esp8266-camera --target upload
-;   pio run -e esp32-mqtt --target upload
+;   pio run -e seeed_xiao_esp32c6 --target upload
 ;
 ; Filesystem:
 ;   pio run -e esp8266-camera --target uploadfs
-;   pio run -e esp32-mqtt --target uploadfs
+;   pio run -e seeed_xiao_esp32c6 --target uploadfs
 
 [common]
 lib_deps =
@@ -43,25 +43,34 @@ build_flags = ${common.build_flags}
 board_build.flash_mode = dio
 board_build.f_cpu = 160000000L
 build_src_filter =
-    +<../lib/>
-    +<esp8266-camera-bridge.cpp>
     -<*.cpp>
+    +<esp8266-camera-bridge.cpp>
+    +<../lib/>
 
-; ── ESP32: MQTT Bridge ─────────────────────────────────────
-; Flashed onto ESP32 Dev Board. Connects to travel router,
-; publishes MQTT to Pi hub. Reads camera status from ESP8266
-; over UART2 (RX16/TX17). No direct camera communication.
+; ── XIAO ESP32-C6: MQTT Bridge ─────────────────────────────
+; Flashed onto a Seeed Studio XIAO ESP32-C6. Connects to the
+; travel router, publishes MQTT to the Pi hub. Reads camera
+; status from the ESP-01S over UART (Serial1: RX=D7, TX=D6).
+; No direct camera communication.
+;
+; ESP32-C6 requires the pioarduino fork of platform-espressif32
+; (mainline espressif32 lagged on the Arduino-core 3.x the C6
+; needs). USB-CDC-on-boot is required for Serial over native USB.
+;
+; Upload fallback if it can't connect: hold B (BOOT), tap
+; R (RESET), release B, then re-run upload.
 
-[env:esp32-mqtt]
-platform = espressif32
-board = esp32dev
+[env:seeed_xiao_esp32c6]
+platform = https://github.com/pioarduino/platform-espressif32/releases/download/stable/platform-espressif32.zip
+board = seeed_xiao_esp32c6
 framework = arduino
 monitor_speed = 115200
-upload_speed = 921600
 lib_deps = ${common.lib_deps}
 build_flags = ${common.build_flags}
     -D CONFIG_ARDUINO_LOOP_STACK_SIZE=8192
+    -D ARDUINO_USB_MODE=1
+    -D ARDUINO_USB_CDC_ON_BOOT=1
 build_src_filter =
-    +<../lib/>
-    +<esp32-mqtt-bridge.cpp>
     -<*.cpp>
+    +<esp32-mqtt-bridge.cpp>
+    +<../lib/>