feat: ground truth labeling pipeline
- Create solver/datagen/labeling.py with label_assembly() function
- Add dataclasses: ConstraintLabel, JointLabel, BodyDofLabel,
AssemblyLabel, AssemblyLabels
- Per-constraint labels: pebble_independent + jacobian_independent
- Per-joint labels: aggregated independent/redundant/total counts
- Per-body DOF: translational + rotational from nullspace projection
- Assembly label: classification, total_dof, has_degeneracy flag
- AssemblyLabels.to_dict() for JSON-serializable output
- Integrate into generate_training_batch (adds 'labels' field)
- Export AssemblyLabels and label_assembly from datagen package
- Add 25 labeling tests + 1 batch structure test (184 total)
Closes #9
forbes-0023
8a49f8ef40