CRUX M6: Alignment — 6 missing stories

Parent: #917

## Scope
RLHF / DPO / PPO-style alignment primitives in aprender-train.

## Contracts in scope
- `contracts/crux-D-{03,04,17,18,19,20}-v1.yaml`

## Exit criteria
- 6 contracts promoted `missing` → `supported`
- entrenar exposes alignment training loop with falsification tests

## Dependencies
- entrenar training infrastructure
- Blocks on: M2 #919 (for scale)
