IMU signals in the car's frame (attitude, gravity-free g, yaw rate) on yukart/imu/<signal>

The WT61's publisher owns mounting and gravity; displays get car-frame values.

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

schema/topics.json

@@ -29,6 +29,13 @@
       "publisher": "gnss",
       "description": "The receiver's fix as single values, one topic per signal, named exactly as in signals.json (source=gnss). For displays; yukart/gnss/fix stays the composite for navigation."
     },
+    "yukart/imu/<signal>": {
+      "payload": "envelope",
+      "retained": true,
+      "rate": "10-50/s",
+      "publisher": "imu",
+      "description": "The IMU (WT61) in the car's frame: attitude, gravity-free acceleration, yaw rate; one topic per signal, named exactly as in signals.json (source=imu)."
+    },
     "yukart/host/<signal>": {
       "payload": "envelope",
       "retained": true,