feat(IMU): Add IMU project README

doc/architecture/IMU/README.md

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+# IMU (MPU6886) ESP-IDF
+=====================
+
+Small ESP-IDF project that reads an MPU6886 IMU over I2C and publishes readings over MQTT.
+Includes an option to run WiFi in STA (client) or AP (access point) mode.
+
+### Features
+
+- I2C communication functions for MPU6886 (accelerometer, gyro, temp)
+- MQTT client using `esp-mqtt`
+- WiFi: STA or AP mode selectable at build time
+- AP mode defaults to static IP 192.168.10.1/24 (configurable in code)
+- MQTT, Serial toggle using the boot button on the esp32
+
+## Quick start
+
+### Prerequisites
+
+- ESP-IDF installed and activated (the repo was developed with ESP-IDF v5.x).
+- Toolchain and Python deps per ESP-IDF instructions.
+
+### Build and flash
+
+From the IMU root:
+
+```bash
+# configure project options
+idf.py menuconfig
+
+# build
+idf.py build
+
+# flash and monitor (set your serial port)
+idf.py -p /dev/ttyUSB0 flash monitor
+```
+
+## Project Configuration (menuconfig)
+
+Open `idf.py menuconfig` and there you will see the `ESP32 IMU Project Configuration` submenu where you can configure specific data for I2C, MQTT and WIFI
+
+### I2C
+---
+In the `I2C Master configuration` submenu there are three options:
+- SCL GPIO Num  - Default 21 - GPIO used for I2C SDA
+- SDA GPIO Num  - Default 22 - GPIO used for I2C SCL
+- Master Frequency - Default 100000hz - I2C bus speed
+
+### MQTT
+
+- Broker URI - Default: `"mqtt://192.168.4.2:1883"`
+- MQTT Topic - Default: `"esp32/imu"`
+
+### WiFi / network
+
+- Toggle AP mode at build time by enabling `CONFIG_WIFI_AP_MODE` in menuconfig. If enabled the firmware will start as an access point on boot; if disabled the device starts as a station and attempts to connect to the configured SSID using the set Password
+- Toggle AP mode swaps the available settings when toggled between AP conf- and station configurations respectively
+- AP credentials can be set with `WiFi AP SSID` and `Wifi AP Password`.
+- Station credentials can be set with `WiFi SSID` and `WiFi Password`.
+
+### AP IP / DHCP
+
+- The AP is configured by default to use a static IP of `192.168.4.1` with netmask `255.255.255.0` and a DHCP server is started so clients receive addresses on that subnet.
+
+## Notes about units and macros
+
+- The macro `SF_RAD_S` (in `main/mpu6886.h`) is the degrees-to-radians conversion factor: PI/180. Gyroscope readings are converted from degrees/sec to radians/sec using this scale.
+- Sensitivity scale constants (e.g. `ACCEL_SO_2G`, `GYRO_SO_250DPS`) are used to convert raw sensor counts to physical units.
+
+## Troubleshooting
+
+- "`MQTT_TOPIC` undefined" at build time: run menuconfig
+- "I2C init failed": confirm SDA/SCL pins in menuconfig or wiring.
+
+## Files of interest
+
+- `main/mpu6886.c` - main application, WiFi init, MQTT publish loop
+- `main/mpu6886.h` - sensor constants and types