firmware: add XIAO C6 sensor node

docs/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- Added standalone XIAO ESP32-C6 BNO086/MAX-M10S sensor-node firmware and a versioned RS485 telemetry contract without changing Cargo `/api/v1` or JBD/NimBLE behavior.
+
+## Unreleased
+
 ### Added
 - Cargo ESP local OTA firmware upload page at `/ota`.
 - Cargo ESP OTA API endpoint at `POST /api/v1/system/ota`.

docs/README.md

@@ -53,5 +53,6 @@ The LILYGO T-Relay-S3 6-way board has been ordered as a future Cargo ESP candida
 Current development remains on the 2-channel relay board while the API and docs are prepared for output-count-agnostic operation.
 
 - `api-v1-audit.md` - Audit of legacy API paths vs `/api/v1` namespace.
+- `SENSOR_NODE_PROTOCOL.md` - XIAO ESP32-C6 BNO086/MAX-M10S RS485 telemetry contract.
 
 - `waveshare-5b-bringup-checklist.md` - Bench bring-up checklist for the Waveshare ESP32-S3 Touch LCD 5B dashboard.

docs/SENSOR_NODE_PROTOCOL.md

@@ -0,0 +1,17 @@
+# XIAO ESP32-C6 Sensor Node Protocol
+
+The XIAO sensor node reads the BNO086 and MAX-M10S and publishes vehicle telemetry over half-duplex RS485 at 115200 8-N-1, one JSON object per line. It owns no control state. Cargo remains the controller, HTTP server, and `/api/v1` source of truth; JBD/NimBLE behavior is unchanged.
+
+The expansion-board pins are D2 driver enable, D4 TX, and D5 RX. Only physical bus ends should be terminated. Confirm A/B polarity because vendor labels vary.
+
+## Schema version 1
+
+`sensor_status` includes `node_id`, `firmware_version`, `uptime_ms`, and:
+
+- `imu`: `online`, `valid`, `accuracy`, quaternion, yaw, pitch, and roll in degrees.
+- `gps`: `online`, `fix`, satellites, latitude, longitude, altitude meters, speed kph, course degrees, and structured UTC.
+
+Consumers must check `valid` or `fix` before displaying measurements and ignore unknown fields. The native breakout coordinate frame is reported initially; installation-specific axis remapping should follow physical calibration.
+
+This link does not replace dashboard-to-Cargo WiFi/HTTP. A future dashboard receiver may display this telemetry without moving relay, BMS, alarm, or configuration authority away from Cargo.
+