Fetches a verified .rdt from the weld planner by digest, checks it, and prepares it on rt-control.

Load runs these checks in order, before any byte reaches the robot:

1. The identity matches the header (dense_identity_mismatch). 2. The plan's robot_cell matches the machine (robot_cell_missing, robot_cell_mismatch, robot_cell_unavailable). 3. Every configured axis has valid Home (home_required). 4. OLP acquires the lease and calls prepare_program. rt-control applies its admission checks. 5. The prepared identity, axis mask and process requirement match (program_identity_or_process_mismatch).

Other errors: 400 dense_identity_invalid, 400 dense_trajectory_invalid, 404 dense_blob_not_found, 503 dense_blob_source_unavailable, 409 program_already_executing, and rt-control reasons with an optional limit_violation.