Implement the engine spine: claim, outbox, machine, API

Contracts first (T02–T04): approval claim schema with issuer, freshness,
and binding digest; local transactional outbox wire; load-bearing cadence
as heartbeat or reconciliation (layer.yaml declared).

Then the object (T06–T08): SQLite closed state machine, CAS supersession,
distinct-approver fail-closed, revocation without holder cooperation,
outbox insert in the same transaction. Tests fail the mutation when
emission fails, and revoke while the drain sink is down.

Introspection GET /v1/approvals/{id}/claim is a PIP fact, not a decision.
No public consume (T05 waits on GH-WP-0002-T06). Canon T-06 coverage for
wrong binding, expiry, revoke, and supersede.

FLEX-WP-0017 T03 is unblocked on this object; T05 remains blocked only on
consumption ordering.

Assistant: grok
Assistant-Session: 01a04ceb-2057-7e20-b0f9-c282964d5dd9

docs/approval-claim.md

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+# Approval claim contract
+
+**Schema:** [`../schemas/approval_claim.schema.json`](../schemas/approval_claim.schema.json)
+**Version:** 0.1
+**Issuer:** `approval-engine`
+**Consumer:** `access-engine` (`flex-auth` until the governed rename)
+
+This is the input claim `access-engine` consumes under statute §6.2. It is a
+fact about an approval object. It is **not a decision**. An implementer can
+satisfy this document without reading this engine's source.
+
+Yields to the Taxonomy request-claim schema (statute §17) when that artifact
+exists and is assented. This local shape is not permanent.
+
+## Fetch
+
+```text
+GET /v1/approvals/{id}/claim
+```
+
+Fail-closed when this engine's store is unavailable (HTTP 503). An `audit-core`
+outage does not affect this read.
+
+There is no `/v1/check`, no `/authorize`, and no field named `effect`,
+`decision`, `allow`, or `deny`. If a response contains those, it is out of
+contract.
+
+## What the claim carries
+
+| Field | Why |
+| --- | --- |
+| `approval_id` | Reconstructable from the decision record. |
+| `binding.digest` | Distinguishes *approved* from *approved for this exact request*. |
+| `binding.pdp_digest` | Optional. When recorded at issue, compare to `NewDecisionBinding.request_digest`. |
+| `issuer` | Always `approval-engine`. |
+| `freshness` | So the PDP can state a deadline for this input class (§9.7.2), not a single fiction covering every source. |
+| `valid_now` | Current-state predicate. Not permission. |
+| `reason_code` | Why `valid_now` is false, when it is. |
+
+`valid_now` is true only when all of:
+
+1. enough distinct authenticated approvers have been recorded;
+2. now is inside `validity.not_before` … `validity.expires_at`;
+3. the object is not consumed, superseded, revoked, or expired.
+
+Holding a claim with `valid_now: true` is not authority to act. It is one
+input the decision point weighs.
+
+## Canonical binding digest
+
+The native digest is:
+
+```text
+sha256: + hex( SHA-256( canonical_json({action, actor, principal, purpose, target}) ) )
+```
+
+`canonical_json` is UTF-8 JSON with sorted keys at every object level and no
+insignificant whitespace (`separators=(',', ':')`). `target` is an object; its
+keys are sorted too.
+
+Wrong-action, wrong-target, and wrong-scope are distinguishable because they
+change that JSON and therefore the digest. A decision rendered against
+approval A for request R cannot be replayed for request R' if the consumer
+compares digests.
+
+### Mapping from a flex-auth CheckRequest
+
+| Claim binding | CheckRequest |
+| --- | --- |
+| `action` | `action` |
+| `target` | `resource` (object) |
+| `actor` | `subject.id` |
+| `principal` | `subject.attributes.principal` if present, else `subject.id` |
+| `purpose` | `context.purpose` |
+
+Go's `json.Marshal` of a `CheckRequest` is **not** this canonical JSON (field
+order and `omitempty` differ). Do not hash a CheckRequest with this function
+and expect it to equal `NewDecisionBinding.request_digest`.
+
+When the issuer recorded the PDP digest at issue time, it is in
+`binding.pdp_digest`. **Prefer that comparison** at decision time:
+
+```text
+claim.binding.pdp_digest == decision.binding.request_digest
+```
+
+and still require `claim.approval_id` to match the approval named on the
+request. Native `binding.digest` remains the identifier of *this engine's*
+binding, and is what T-06 uses when no PDP digest was recorded.
+
+## Freshness
+
+Default TTL is **30 seconds** from `observed_at`. `not_after` is
+`observed_at + ttl_seconds`. A PDP that needs a different deadline for the
+approval input class states it in its own provenance; it must not invent
+freshness this engine did not publish.
+
+A claim used after `freshness.not_after` is stale. Stale is not the same as
+`valid_now: false` — the object may still be valid; the *observation* is old.
+Re-fetch.
+
+## Required verification (consumer)
+
+A production consumer of this claim, before treating it as an input, checks:
+
+1. The claim resolved from this engine; an outage fails the action closed.
+2. `issuer` is `approval-engine`.
+3. `valid_now` is true and `consumed` is false.
+4. `binding.digest` equals the digest of the binding the consumer computed
+   from the proposed action, **or** `binding.pdp_digest` equals the
+   `NewDecisionBinding` digest of that request.
+5. `freshness.not_after` is still in the future.
+6. `reason_code` is `ok`.
+
+Local fixtures, workplan ids, and prose are not this claim.
+
+## Examples
+
+See [`../examples/claim.valid.json`](../examples/claim.valid.json) and
+[`../examples/claim.revoked.json`](../examples/claim.revoked.json).