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/__init__.py

@@ -0,0 +1,24 @@
+"""approval-engine — PIP for the approval object."""
+
+from .binding import binding_digest, canonical_binding
+from .errors import (
+    ApprovalError,
+    Conflict,
+    DuplicateApprover,
+    NotFound,
+    StoreUnavailable,
+    Unprocessable,
+)
+from .store import Engine
+
+__all__ = [
+    "Engine",
+    "ApprovalError",
+    "Conflict",
+    "DuplicateApprover",
+    "NotFound",
+    "StoreUnavailable",
+    "Unprocessable",
+    "binding_digest",
+    "canonical_binding",
+]