Add two spec-prescribed tests that the implementation plan missed:
- Create_proceeds_when_client_possession_snapshot_disagrees_with_server
- Protect_then_load_index_emits_is_protected_one
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Five wire-level integration tests covering: flag set, round-trip unset,
401 without auth, 400 unknown_card, and empty-object response shape.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>