[EPIC E1] Minimal Product Contract Kernel

## Outcome

one small backend-neutral vocabulary can express the caller, reusable role, requested work, policy decision, issued authority, claim, and evidence.

## Delivery contract

- **Epic:** E1
- **Priority:** P0
- **Wave:** Wave A - Authority spine
- **Milestone:** Mid-term - Governed Vertical Slice
- **Area:** area:contracts

## Completion evidence

the fixtures compile/parse through shared types and every invalid case is rejected before any backend-specific code is involved.

## Child delivery

Implementation tickets are attached as native GitHub sub-issues. Closing child PRs is not sufficient: this Epic closes only when the completion evidence above is reproducible.

## Product constraints

- Preserve the claim-scoped, backend-neutral architecture contract.
- Do not broaden this Epic into deferred platform work.
- Gate failures must narrow scope or repair the harness before more implementation is added.

No files, tests, or entry points are named. Start by reading the attached child sub-issues and locating the fixtures and shared types they reference; done means fixtures compile or parse through backend-neutral types and every invalid case is rejected before backend-specific code runs.

Written by the indexing model from the issue text.