feat: add PlatformIO ESP32 firmware with dual-STA + MQTT + GoPro control

firmware/
├── platformio.ini     — ESP32 (esp32dev), PubSubClient + ArduinoJson
├── src/main.cpp       — Full camera node firmware (360 lines)
│   ├── SPIFFS config persistence
│   ├── Dual Wi-Fi STA (travel router + GoPro AP)
│   ├── GoPro Hero 3 HTTP API (start/stop/status)
│   ├── 60-byte binary status blob parser
│   ├── MQTT per contract (status QoS1, heartbeat QoS1, announce QoS2)
│   ├── Command subscription (start/stop/reboot)
│   ├── Auto-registration (announce → hub assigns cam-NNN)
│   ├── Heartbeat every 60s, status every 30s
│   ├── LED status indicator
│   └── Exponential backoff reconnection
├── data/config.json   — Default SPIFFS config template
└── README.md          — Quick start, config reference, troubleshooting

firmware/README.md

@@ -0,0 +1,118 @@
+# RemoteRig — ESP32 Camera Node Firmware
+
+> **Platform:** PlatformIO (esp32dev) | **Framework:** Arduino
+> **MQTT Contract:** [docs/MQTT_CONTRACT.md](../docs/MQTT_CONTRACT.md)
+> **Hardware:** [hardware/README.md](../hardware/README.md)
+
+## Quick Start
+
+```bash
+# Install PlatformIO (if not already)
+pip install platformio
+
+# Build
+cd firmware
+pio run
+
+# Upload to ESP32 (USB connected)
+pio run --target upload
+
+# Upload SPIFFS config (first time only, or after config changes)
+pio run --target uploadfs
+
+# Serial monitor
+pio device monitor
+```
+
+## Configuration
+
+The ESP32 stores configuration in SPIFFS (`data/config.json`):
+
+| Key | Default | Description |
+|-----|---------|-------------|
+| `wifi_ssid` | `"RemoteRig"` | Travel router SSID |
+| `wifi_password` | `""` | Travel router password |
+| `camera_ssid` | `"GOPRO-BP-"` | GoPro Wi-Fi AP prefix (auto-discovered) |
+| `camera_password` | `"goprohero"` | GoPro Wi-Fi password |
+| `mqtt_broker` | `"192.168.4.10"` | Pi Zero 2 W static IP |
+| `mqtt_port` | `1883` | Mosquitto port |
+| `camera_id` | `""` | Assigned by hub on first announce (leave empty) |
+| `poll_interval_sec` | `30` | GoPro status poll frequency |
+| `heartbeat_interval_sec` | `60` | MQTT heartbeat frequency |
+
+**First boot:** Leave `camera_id` empty. The ESP32 will auto-announce to the hub, which assigns a `cam-NNN` ID. The assigned ID is saved to SPIFFS automatically.
+
+## LED Status Codes
+
+| Pattern | Meaning |
+|---------|---------|
+| Slow blink (1s) | Connected to router + MQTT, normal operation |
+| Fast blink (200ms) | No Wi-Fi connection — reconnecting |
+| Solid on | Connected but GoPro unreachable |
+| Off | Boot/shutdown |
+
+## Architecture
+
+```
+┌──────────────────────────────────────────┐
+│              ESP32 (Arduino)             │
+│                                          │
+│  ┌──────────┐  ┌──────────┐  ┌────────┐  │
+│  │ WiFi STA │  │ WiFi STA │  │  MQTT  │  │
+│  │ (Router) │  │ (GoPro)  │  │ Client │  │
+│  └────┬─────┘  └────┬─────┘  └───┬────┘  │
+│       │             │            │       │
+│       │    ┌────────┘            │       │
+│       ▼    ▼                     ▼       │
+│  ┌─────────────────────────────────┐     │
+│  │         Main Loop               │     │
+│  │  Every 30s:                    │     │
+│  │    HTTP GET GoPro status       │     │
+│  │    Parse 60-byte blob          │     │
+│  │    MQTT publish status         │     │
+│  │  Every 60s:                    │     │
+│  │    MQTT publish heartbeat      │     │
+│  └─────────────────────────────────┘     │
+│                                          │
+│  SPIFFS: /config.json (persistent)       │
+└──────────────────────────────────────────┘
+```
+
+## Boot Sequence
+
+1. Load config from SPIFFS
+2. Connect to travel router Wi-Fi (STA mode)
+3. Connect to GoPro AP Wi-Fi (STA mode — simultaneous)
+4. Connect to MQTT broker (192.168.4.10)
+5. If no `camera_id` → publish announce → hub registers us
+6. Subscribe to `remoterig/cameras/{camera_id}/command`
+7. Enter main loop
+
+## GoPro API Notes (Hero 3 Black/Silver)
+
+- **IP:** Always `10.5.5.1` (GoPro's own AP)
+- **Status endpoint:** `GET /bacpac/SH?t={password}&p=%01`
+- **Start recording:** `GET /bacpac/SH?t={password}&p=%01` (mode byte = 1)
+- **Stop recording:** `GET /bacpac/SH?t={password}&p=%00` (mode byte = 0)
+- **Get password:** `GET /bacpac/sd` (no auth, returns plain text)
+- **Status blob:** 60 bytes binary — see `parseStatus()` in main.cpp for field offsets
+
+## ESP8266 Compatibility
+
+To target ESP8266 instead:
+1. Change `platformio.ini`: `board = d1_mini` under `[env:d1_mini]`
+2. Change `WiFi.h` → `ESP8266WiFi.h`
+3. ESP8266 doesn't do true simultaneous STA — use single STA to travel router, HTTP to GoPro via router bridge
+4. SPIFFS → LittleFS on some boards
+
+ESP32 is recommended for dual-STA capability.
+
+## Troubleshooting
+
+| Symptom | Check |
+|---------|-------|
+| No serial output | Baud rate: 115200. Hold BOOT, press EN, release BOOT for flash mode |
+| Can't connect to router | Verify SSID/password in SPIFFS config, check router DHCP range |
+| GoPro unreachable | GoPro must be ON and Wi-Fi enabled. Password defaults to "goprohero" |
+| MQTT connect fails | Verify Mosquitto running on Pi: `systemctl status mosquitto` |
+| Camera never registers | Watch serial for "announce" message, check hub logs for registration |