imu: the WT61 on any USB-UART adapter, in the car's frame, on yukart/imu/*

Finds the module itself (every non-modem adapter, 115200 then 9600), so the
adapter can be swapped. Mounting by --forward/--right; attitude from a
complementary filter on gravity (gyro through motion, accelerometer near 1 g);
gravity removed from acceleration; yaw rate left-positive. Never writes to
the module. dialout via SupplementaryGroups.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

README.md

@@ -10,7 +10,7 @@ This file serves as the minimalist portal to other docs
 ## What this repo is about
 
 The acquisition and processing cluster on the Orin, developed and deployed as one
-unit: `recorder`, `decoder`, `gnss`, `orin-status` and `obd-poller` now; `can-bridge` as it is
+unit: `recorder`, `decoder`, `gnss`, `imu`, `orin-status` and `obd-poller` now; `can-bridge` as it is
 needed.
 
 | Binary | Installed as | Does |
@@ -20,6 +20,7 @@ needed.
 | `bus-bench` | not installed: a test tool | Raw CAN and decoded-signal load over MQTT, measured: loss, latency, broker CPU ([report](docs/impl/reports/BUS-BENCH.md)) |
 | `obd-poller` | `yukart-obd-poller.service` | OBD-II mode 01 on the K-line through the Bluetooth ELM327 (address in `/etc/default/yukart-elm`), the single writer to the car; never on CAN. The `obd_*` signals, calibration references (`yukart/vehicle/*`): rpm, speed, throttle, load, MAF, timing, O2 every round; temperatures and trims every 5 s. Lets go of the adapter after a minute of ECU silence |
 | `gnss` | `yukart-gnss.service` | gpsd's fix as single values on `yukart/gnss/*`: fix, speed, heading, position, altitude, satellites |
+| `imu` | `yukart-imu.service` | The WT61 (plus) on any USB-UART adapter, found by itself (adapter and speed), turned into the car's frame (`--forward`, `--right`): roll, pitch, gravity-free longitudinal and lateral g, yaw rate on `yukart/imu/*`, at most every 40 ms. Never writes to the module |
 | `orin-status` | `yukart-orin-status.service` | The Orin's health on the bus, once a second: temperatures, CPU/GPU load, memory, power, fan, WiFi (with SSID)/LTE/tablet links, the CAN interface, the ELM's Bluetooth link, UPS battery (`yukart/host/*`) |
 
 ```bash
@@ -32,6 +33,7 @@ zstdcat can-*.log.zst | yukart-decoder --replay   # what it would publish for a
 yukart-obd-poller --elm ADDR --once        # on the Orin, service stopped: adapter, protocol,
                                            # supported PIDs and one value each
 yukart-gnss --once                         # the next fix as JSON, no broker
+yukart-imu --once                          # find the WT61, 3 s of readings as JSON, no broker
 yukart-orin-status --once                  # on an Orin: one reading as JSON, no broker
 yukart-recorder ctl status                 # on the Orin
 yukart-recorder ctl start "cold start"     # manual session, ends only on stop