feat(data): optionally export arm-local YAM end-effector poses

[sawhney17]

Summary

• Adds an opt-in --derive-ee-poses flag to export_mcap.py and export_hf_task.py.
• Writes a separate end_effector_poses.npz sidecar with observed/commanded left/right grasp-site poses, aligned timestamps, and a validity mask.
• Leaves states_actions.bin, videos, metadata shape, and all legacy outputs unchanged when the flag is off.
• Uses the official bundled I2RT YAM model and records model SHA-256, MuJoCo version, frame, units, quaternion convention, alignment method, and mask assignments.
• Emits only arm-local poses. It does not guess the transform between physical arm bases.
• Keeps missing, malformed, or non-finite joint samples as NaN with an unset validity bit; legacy zero-padding is never passed to FK.

Why

Issue #13 asks for an official FK procedure for raw episodes whose EEF pose fields are empty. I independently audited all 40 ungated raw MCAPs in the pinned Voxel51 preview:

• 314,081 of 641,593 exact arm records have an empty pose field.
• All 314,081 retain six finite joints and are recoverable arm-locally.
• The YAM chain validates against 327,512 intact records.
• Intact episodes expose two right-base offsets, -0.61 m and -0.80 m. Hard-coding the common offset creates a 19 cm error on 16,118 valid right-arm records, so this patch intentionally refuses a universal shared-frame transform.

Reproducible audit, independent cross-check, recovery artifact, and interactive proof:
https://github.com/sawhney17/abc-geometry-recovery

Data contract

The optional NPZ uses schema abc.end_effector_poses.v1. It contains four (N, 7) float64 arrays in [x, y, z, qw, qx, qy, qz] order, timestamp_ns, and a uint8 valid_mask. Bits 0-3 map to left state, right state, left action, and right action.

Validation

• 7 focused tests pass in a minimal Python 3.12 environment.
• Literal public left/right fixtures match the bundled official model to 1e-12.
• Missing and malformed topics remain NaN and unmarked.
• Causal alignment and all four validity bits are covered.
• Sidecar publication is atomic and metadata is JSON-serializable.
• Historical three-item export jobs remain compatible; wrapper forwarding is opt-in only.

Related to #13. This addresses the FK/export portion only; it does not claim to solve the camera calibration requests in that issue.