Define the rubric, criteria, evidence, and judgement data model

Specify the core domain model for rubrics, criteria, evidence, observations, provisional judgements, marks, and feedback.

### Deliverables
- Entity and relationship definitions
- Versioning rules for rubrics and assessment artefacts
- Boundaries between observed evidence, model suggestions, and educator judgement

### Done when
The model supports reproducibility without conflating AI output with verified judgement.

The issue names no files, tests, or entry points. Begin by mapping the requested entities and relationships, then define versioning and the boundary between observed evidence, model suggestions, and educator judgement. Done means the model supports reproducibility without treating AI output as verified judgement.

Written by the indexing model from the issue text.