fix: apply IMU extrinsic in get_body_cloud / get_body_cloud_down

[leshy]

get_body_cloud() / get_body_cloud_down() returned the undistorted scan in the raw LiDAR/sensor frame, while odometry/tf reports the IMU pose — downstream registration is off by the extrinsic (~5 cm on a Mid-360).

Transform each point LiDAR→IMU into a copy via the existing RGBpointBodyLidarToIMU (now const), mirroring vanilla FAST-LIO publish_frame_body / cloud_registered_body. feats_undistort / feats_down_body are untouched — the EKF still reads them.

The get_body_cloud groundwork is otherwise kept as-is. dimos-side flake bump follows after merge.

[aur3l14no]

Could we also add a get_world_cloud_down() getter here?

For downstream consumers such as navigation / PGO, it is useful to publish a registered world-frame scan without forcing the full dense cloud path. get_world_cloud() already exists, and this would mirror the existing get_body_cloud_down() behavior using FAST-LIO's downsampled scan.

I have a small branch that adds it on top of this work for reference:
https://github.com/aur3l14no/dimos-module-fastlio2/tree/feat/world-cloud-down

Since this PR is still unmerged, I wasn't sure whether you'd prefer folding this into this branch or handling it later as a stacked/follow-up PR.