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

approval_engine/errors.py

@@ -0,0 +1,28 @@
+class ApprovalError(Exception):
+    http_status = 400
+    reason_code = "error"
+
+
+class NotFound(ApprovalError):
+    http_status = 404
+    reason_code = "not_found"
+
+
+class Conflict(ApprovalError):
+    http_status = 409
+    reason_code = "conflict"
+
+
+class DuplicateApprover(ApprovalError):
+    http_status = 409
+    reason_code = "duplicate_approver"
+
+
+class Unprocessable(ApprovalError):
+    http_status = 422
+    reason_code = "unprocessable"
+
+
+class StoreUnavailable(ApprovalError):
+    http_status = 503
+    reason_code = "store_unavailable"