feat(transformations): configurable anchor frame + self-describing compensation result

[janickm]

Summary

Follow-up to the reference-timestamp work (#146). Make the motion-compensation anchor frame an explicit parameter and expose the frame IDs in the result so a compensation is fully self-describing.

Changes

• MotionCompensator.motion_compensate_points / motion_decompensate_points gain anchor_frame_id: str = "world". The anchor is the common frame the per-point transforms are composed through; the default preserves the previous hard-coded "world" behavior. Decompensation must use the same anchor as compensation (documented in both docstrings).
• MotionCompensationResult now carries:
  • reference_frame_id -- the sensor frame the compensated points are expressed in (equals the sensor_id)
  • anchor_frame_id -- the common anchor frame used to compose the transforms
  • alongside the existing xyz_s_reftime / xyz_e_reftime / reference_timestamp_us.
• MotionCompensator.from_sensor_rig gains anchor_frame_id: str = "world" and rig_frame_id: str = "rig" so the pose-graph node names are no longer hard-coded.

All existing callers keep the defaults, so behavior is unchanged.

Note on valid anchors

A valid anchor must be a frame the sensor actually moves relative to within the frame -- i.e. the dynamic/global frame, or one related to it by a static transform (e.g. world / world_global). The rig frame is not a valid anchor, because the sensor->rig extrinsic is static, so compensation through it would see no motion. This is reflected in the tests.

Testing

bazel test //ncore/impl/common:test_3_11 \
            //ncore/impl/data/v4:pytest_compat_3_11 \
            //ncore/impl/sensors:pytest_lidar_3_11

New tests:

• test_result_exposes_frame_ids -- result records the reference (sensor) and anchor frame IDs, including on the empty-input path.
• test_compensation_independent_of_static_anchor_choice -- anchors related by a static transform (world / world_global) yield identical compensated points; round-trip with the non-default anchor recovers the input.
• test_from_sensor_rig_custom_frame_names -- from_sensor_rig honors custom anchor/rig node names with a clean compensate/decompensate round-trip.

All of //ncore/... and the affected //tools/... packages build and type-lint cleanly; the existing compensator/compat/lidar tests still pass.

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[janickm]

/ok to test a5263b9