feat: dual-board architecture — ESP8266 camera bridge + ESP32 MQTT bridge

Complete rewrite of firmware into two dedicated boards per camera node:

ESP8266 (Camera Bridge):
- Connects ONLY to GoPro AP — polls status, sends over UART
- Zero network switching, zero MQTT
- HTTP GET /bacpac/SH for status, start/stop
- JSON-per-line UART protocol to ESP32

ESP32 (MQTT Bridge):
- Connects ONLY to travel router — MQTT to Pi hub
- Reads status from ESP8266 over UART2 (RX16/TX17)
- Auto-registration, heartbeat, command forwarding
- Zero camera communication

UART Protocol: JSON-per-line at 115200 8N1
  ESP8266→ESP32: status/ack/pong/error
  ESP32→ESP8266: cmd (start_recording/stop_recording/ping)

Hardware updates:
- BOM now includes both boards (~4/node)
- 3D case has stacked dual-board compartment
- UART wire channel between board recesses
- Shared 3.3V power rail for both boards

firmware/platformio.ini

@@ -1,17 +1,24 @@
-; RemoteRig — ESP32 + ESP8266 Camera Node Firmware
-; PlatformIO project with dual-target support.
+; RemoteRig — Dual-Board Camera Node Firmware
+; ============================================
+; Each camera node has TWO boards connected via UART:
+;
+;   ESP8266 (Camera Bridge):   Connects to GoPro AP → HTTP status/control
+;   ESP32   (MQTT Bridge):     Connects to travel router → MQTT to hub
+;
+;   ESP8266 ←──UART──→ ESP32
+;   (TX/RX)           (RX16/TX17)
 ;
 ; Build:
-;   pio run -e esp32dev     (ESP32 Dev Board — dual-STA, recommended)
-;   pio run -e esp8266dev   (ESP8266 D1 Mini — time-shared STA)
+;   pio run -e esp8266-camera    (ESP8266 D1 Mini — camera bridge)
+;   pio run -e esp32-mqtt        (ESP32 Dev Board — MQTT bridge)
 ;
 ; Upload:
-;   pio run -e esp32dev --target upload
-;   pio run -e esp8266dev --target upload
+;   pio run -e esp8266-camera --target upload
+;   pio run -e esp32-mqtt --target upload
 ;
-; SPIFFS/LittleFS:
-;   pio run -e esp32dev --target uploadfs
-;   pio run -e esp8266dev --target uploadfs
+; Filesystem:
+;   pio run -e esp8266-camera --target uploadfs
+;   pio run -e esp32-mqtt --target uploadfs
 
 [common]
 lib_deps =
@@ -20,17 +27,11 @@ lib_deps =
 build_flags =
     -D CORE_DEBUG_LEVEL=0
 
-[env:esp32dev]
-platform = espressif32
-board = esp32dev
-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
+; ── ESP8266: Camera Bridge ──────────────────────────────────
+; Flashed onto D1 Mini. Talks to GoPro over Wi-Fi, relays to
+; ESP32 over UART (TX/RX pins). No MQTT, no router connection.
 
-[env:esp8266dev]
+[env:esp8266-camera]
 platform = espressif8266
 board = d1_mini
 framework = arduino
@@ -39,6 +40,28 @@ upload_speed = 921600
 lib_deps = ${common.lib_deps}
 build_flags = ${common.build_flags}
     -D PIO_FRAMEWORK_ARDUINO_MMU_CACHE16_IRAM48_SECHEAP_SHARED
-    -D CONFIG_ARDUINO_LOOP_STACK_SIZE=8192
 board_build.flash_mode = dio
 board_build.f_cpu = 160000000L
+build_src_filter =
+    +<../lib/>
+    +<esp8266-camera-bridge.cpp>
+    -<*.cpp>
+
+; ── 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.
+
+[env:esp32-mqtt]
+platform = espressif32
+board = esp32dev
+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
+build_src_filter =
+    +<../lib/>
+    +<esp32-mqtt-bridge.cpp>
+    -<*.cpp>