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

docs/BOUNDARY.md

@@ -37,7 +37,7 @@ The two channels from decision 24 share the socket but keep different guarantees
 | `ack` | reply to a `cmd` | `id`, `ok`, `detail`, `dup` (true when answered from the cache) |
 
 With `source: bus`, `signals` is the newest envelope of every signal svc-gw has
-seen on `yukart/vehicle/+`, `yukart/host/+` and `yukart/gnss/+`. svc-gw marks one
+seen on `yukart/vehicle/+`, `yukart/host/+`, `yukart/gnss/+` and `yukart/imu/+`. svc-gw marks one
 `quality: "stale"` when it has not been refreshed for 5 s, or is known only from
 the broker's retained copy (which may predate a reboot). A signal once seen is
 never removed.