Phase 1 — Make the three contracts explicit & tested

[rubenhensen]

Goal: turn the three implicit seams into committed, machine-checkable artifacts and guard them in the harness. Directly protects the in-flight condiscon work.

Done when: a deliberate shape change to any seam fails a contract test.

[rubenhensen]

Delivered: the three contracts are pinned and tested — #242 (OpenAPI + JSON Schema + contract examples, deny_unknown_fields), #243 (golden wire fixtures + tripwire), condiscon schema; L1 legs run in encryption4all/postguard-e2e. Follow-on work continues under #247.