docs: complete USB protocol specification (docs/PROTOCOL.md)

docs/PROTOCOL.md

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+# DongLoRa USB Protocol Specification
+
+DongLoRa communicates over USB CDC-ACM (virtual serial port) using
+binary messages. No baud rate configuration is needed — USB handles
+the transport.
+
+**USB identifiers:** VID `1209`, PID `5741`
+
+## Framing
+
+Each message is serialized with [postcard](https://postcard.jamesmunns.com/)
+and framed with [COBS](https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing):
+
+```
+[COBS-encoded postcard bytes] [0x00 sentinel]
+```
+
+COBS guarantees that `0x00` never appears in the encoded data, so the
+sentinel byte unambiguously marks the end of each frame. Maximum frame
+size is 512 bytes. USB packets are chunked at 64 bytes.
+
+## Serialization (postcard)
+
+Postcard is a compact binary format. The subset used by DongLoRa:
+
+| Type | Encoding |
+|------|----------|
+| `u8` | Raw byte |
+| `u16`, `u32` | Varint (LEB128: 7 data bits per byte, MSB = continuation) |
+| `i8`, `i16` | Zigzag then varint (`(n << 1) ^ (n >> bits-1)`) |
+| Enum variant | Varint of 0-based variant index, then variant fields |
+| `Option<T>` | `0x00` = None, `0x01` + T = Some |
+| `Vec<u8, N>` | Varint length, then raw bytes |
+| Struct | Fields in declaration order, no delimiters |
+
+## Commands (Host → Firmware)
+
+| Index | Command | Fields | Description |
+|-------|---------|--------|-------------|
+| 0 | `Ping` | — | Health check. Returns `Pong`. |
+| 1 | `GetConfig` | — | Request current radio config. Returns `Config` or `Error(NotConfigured)`. |
+| 2 | `SetConfig` | `RadioConfig` | Set radio parameters. Validated against hardware limits. Returns `Ok` or `Error(InvalidConfig)`. |
+| 3 | `StartRx` | — | Enter continuous receive mode. Returns `Ok` or `Error`. |
+| 4 | `StopRx` | — | Return to idle. Returns `Ok`. |
+| 5 | `Transmit` | `Option<RadioConfig>`, `Vec<u8, 256>` | Transmit a packet. Optional per-packet config override. Returns `TxDone` or `Error`. Automatically resumes RX if it was active. |
+| 6 | `DisplayOn` | — | Turn on the OLED display. |
+| 7 | `DisplayOff` | — | Turn off the OLED display. |
+
+## Responses (Firmware → Host)
+
+| Index | Response | Fields | Description |
+|-------|----------|--------|-------------|
+| 0 | `Pong` | — | Reply to `Ping`. |
+| 1 | `Config` | `RadioConfig` | Current radio configuration. |
+| 2 | `RxPacket` | `rssi: i16`, `snr: i16`, `payload: Vec<u8, 256>` | Received packet with signal quality. |
+| 3 | `TxDone` | — | Transmission complete. |
+| 4 | `Ok` | — | Command succeeded (SetConfig, StartRx, StopRx). |
+| 5 | `Error` | `ErrorCode` | Command failed. |
+
+## RadioConfig
+
+| Field | Type | Wire encoding | Valid range | Description |
+|-------|------|---------------|-------------|-------------|
+| `freq_hz` | `u32` | varint | 150,000,000 – 960,000,000 | Frequency in Hz |
+| `bw` | `Bandwidth` | varint (0–9) | see table | Signal bandwidth |
+| `sf` | `u8` | varint | 5 – 12 | Spreading factor |
+| `cr` | `u8` | varint | 5 – 8 | Coding rate denominator (5 = CR 4/5) |
+| `sync_word` | `u16` | varint | any | LoRa sync word (e.g. `0x3444`) |
+| `tx_power_dbm` | `i8` | zigzag varint | -9 – +22 | TX power in dBm |
+
+## Bandwidth
+
+| Wire value | Bandwidth |
+|------------|-----------|
+| 0 | 7.8 kHz |
+| 1 | 10.4 kHz |
+| 2 | 15.6 kHz |
+| 3 | 20.8 kHz |
+| 4 | 31.25 kHz |
+| 5 | 41.7 kHz |
+| 6 | 62.5 kHz |
+| 7 | 125 kHz |
+| 8 | 250 kHz |
+| 9 | 500 kHz |
+
+## ErrorCode
+
+| Wire value | Error | Meaning |
+|------------|-------|---------|
+| 0 | InvalidConfig | Radio config validation failed |
+| 1 | RadioBusy | Radio is busy (e.g. RX restart failed) |
+| 2 | TxTimeout | Transmission timed out |
+| 3 | CrcError | CRC error on received packet |
+| 4 | NotConfigured | Command requires SetConfig first |
+| 5 | NoDisplay | Display command sent but no display attached |
+
+## Worked Example
+
+Configure the radio for 915 MHz, 125 kHz BW, SF7, CR 4/5, sync word `0x3444`,
+14 dBm TX power.
+
+### 1. Build the RadioConfig
+
+| Field | Value | Varint bytes |
+|-------|-------|-------------|
+| `freq_hz` | 915,000,000 | `80 84 AF B4 03` |
+| `bw` | 7 (125 kHz) | `07` |
+| `sf` | 7 | `07` |
+| `cr` | 5 (CR 4/5) | `05` |
+| `sync_word` | 0x3444 | `C4 E8 00` |
+| `tx_power_dbm` | 14 → zigzag 28 | `1C` |
+
+### 2. Build the SetConfig command
+
+Prepend the variant index for `SetConfig` (index 2):
+
+```
+02 80 84 AF B4 03 07 07 05 C4 E8 00 1C
+```
+
+### 3. COBS-encode and send
+
+COBS encoding wraps the bytes so `0x00` never appears in the data,
+then appends a `0x00` sentinel:
+
+```
+[COBS-encoded bytes] 00
+```
+
+Write the resulting bytes to the USB serial port. Read back a COBS
+frame, decode it, and the first byte is the response variant index
+(4 = `Ok`).
+
+## Host Implementation Checklist
+
+1. Open the USB serial port (find by VID:PID `1209:5741`)
+2. Implement varint and zigzag encoding/decoding
+3. Implement COBS encode/decode (use a library — available in every language)
+4. Build `SetConfig` with your desired radio parameters
+5. Send it, read back `Ok`
+6. Send `StartRx`, read back `Ok`
+7. Loop: read frames, decode `RxPacket` responses
+8. To transmit: send `Transmit` with payload, read back `TxDone`
+
+See `examples/` for working Python implementations.