[Phase 0] Add field-level provenance metadata for derived fields #339
Description
Type: feature
Suggested labels: phase-0, metadata, lineage
For curated fields that may come from multiple sources, expose field-level provenance metadata (source precedence and derivation rule).
Acceptance criteria:
- Documented field-level provenance spec in repo.
- Metadata artifact generated in build (for example YAML/JSON alongside datapackage metadata).
- At least games curated model includes field-level provenance entries.
Dependencies: #7