imu: raw samples on yukart/raw/imu, recorded as imu.ndjson; calibration from a session

--calibrate SESSION [--write]: down from two level stretches (turning round cancels
the slope), forward from straight pull-aways and stops; saved to
/var/lib/yukart/imu/mount.json (StateDirectory), which the service reloads on change.

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

deploy/yukart-imu.service

@@ -10,7 +10,10 @@ StartLimitIntervalSec=0
 User=mikkeli
 # USB serial adapters belong to dialout: the one right it needs, no sudo.
 SupplementaryGroups=dialout
-# Mounting: add --forward AXIS --right AXIS once the module has its place in the car.
+# /var/lib/yukart/imu, owned by the service user: the measured mounting (mount.json),
+# written by `yukart-imu --calibrate SESSION --write`. Until one exists, the defaults
+# (module flat, Y forward) or --forward/--right apply.
+StateDirectory=yukart/imu
 ExecStart=/usr/local/bin/yukart-imu
 Restart=always
 RestartSec=2