docs: Finalize v0.3, add OTA section to USAGE.md

docs/USAGE.md

@@ -163,6 +163,86 @@ The router firmware uses only LLTF for broader router compatibility. The ESP-NOW
 | `CONFIG_FORCE_GAIN` | `0` | Force AGC/FFT gain to baseline (disabled) |
 | `CONFIG_GAIN_CONTROL` | `1` (auto) | Enable gain compensation (ESP32-S3, C3, C5, C6, C61) |
 
+## Remote Management
+
+### Commands (esp-cmd)
+
+Send commands to individual devices over UDP port 5501:
+
+```bash
+esp-cmd <host> STATUS        # Uptime, heap, RSSI, tx_power, rate, version
+esp-cmd <host> IDENTIFY      # LED solid 5s
+esp-cmd <host> RATE <10-100> # Set ping rate (NVS saved)
+esp-cmd <host> POWER <2-20>  # Set TX power dBm (NVS saved)
+esp-cmd <host> OTA <url>     # Trigger OTA update (prefer esp-ota)
+esp-cmd <host> REBOOT        # Restart device
+```
+
+### Fleet Management (esp-fleet)
+
+Send commands to all sensors in parallel:
+
+```bash
+esp-fleet status             # Query all devices
+esp-fleet identify           # Blink all LEDs
+esp-fleet rate 50            # Set rate on all
+esp-fleet reboot             # Reboot all
+esp-fleet ota                # OTA update all (sequential)
+esp-fleet ota /path/to/fw    # OTA with custom firmware
+```
+
+## OTA Updates
+
+### Overview
+
+Firmware updates are delivered over WiFi using ESP-IDF's `esp_https_ota` with a dual OTA partition layout. The device downloads a new binary from an HTTP server on the Pi, writes it to the inactive OTA slot, and reboots.
+
+### Partition Layout
+
+| Partition | Offset | Size | Purpose |
+|-----------|--------|------|---------|
+| nvs | 0x9000 | 16 KB | NVS config storage |
+| otadata | 0xd000 | 8 KB | OTA boot selection |
+| phy_init | 0xf000 | 4 KB | PHY calibration |
+| ota_0 | 0x10000 | 1920 KB | App slot A |
+| ota_1 | 0x1F0000 | 1920 KB | App slot B |
+
+### Using esp-ota
+
+The `esp-ota` tool handles the full OTA workflow:
+
+```bash
+esp-ota amber-maple.local               # Default build path
+esp-ota amber-maple.local -f custom.bin # Custom firmware
+esp-ota amber-maple.local --no-wait     # Don't verify reboot
+```
+
+**What it does:**
+
+1. Verifies the device is alive (`STATUS`)
+2. Starts a temporary HTTP server on port 8070
+3. Sends `OTA http://<pi-ip>:8070/<firmware>.bin` to the device
+4. Waits for the device to download, flash, reboot
+5. Verifies the device responds with the new version
+
+### Rollback
+
+The bootloader supports automatic rollback on failed updates:
+
+1. New firmware is written to the inactive OTA slot
+2. Device reboots into the new firmware in `PENDING_VERIFY` state
+3. If the firmware boots successfully (WiFi connects, mDNS starts, command listener ready), it marks itself as valid
+4. If the firmware crashes or hangs, the 30s watchdog triggers a reboot and the bootloader rolls back to the previous slot
+
+### First-Time Setup
+
+The initial flash **must be done via USB** because switching from a single-app to a dual-OTA partition table requires erasing the entire flash. After the first USB flash, all subsequent updates can be done via OTA.
+
+```bash
+idf.py -p /dev/ttyUSB0 flash   # First time: USB required
+esp-ota amber-maple.local       # All subsequent updates: OTA
+```
+
 ## Receiving CSI Data on the Pi
 
 Listen on UDP port 5500: