feat: bind the destroy gate to approval_binding_digest and pdp_path

The vocabulary mapping this path was waiting on is not coming: gate-house
rejected it in GH-DEC-2026-008, because a translation can be confidently
wrong and fails open by accepting a claim approved for a different action.
The stronger option arrived instead, and both halves are enforced here.

flex-auth published binding.approval_binding_digest (FLEX-DEC-2026-007) to
fix the circularity this repo reported: a pdp_digest recorded at issue time
can never equal the request_digest of the request that carries the claim in
its hashed context, so with GH-DEC-2026-008 requiring that equality, destroy
would have failed closed forever on a check no correct record could pass.

- authorization.approval_binding_digest implements the published exclusion
  rule, including Go's context,omitempty behaviour when stripping empties
  the context; digest_material drops an empty context for the same reason.
- validate_decision_envelope recomputes the field rather than trusting it,
  refuses a claim-bearing request whose decision records none, and compares
  the claim's digest from step 1 against it -- never against request_digest,
  which still covers the claim so it stays a sound replay identity.
- validate_approval_claim requires binding.pdp_path true before using
  pdp_digest at all. Path intent is never inferred from a digest that
  happens to be present; pre-schema-v3 approvals carry pdp_path false
  regardless of any digest they hold.

Replay fixtures re-vendored from dd3ce4c. The destroy pins moved a second
and final time; approval_binding_digest did not, which is the point. The
fixture now demonstrates the property instead of asserting it: we rederive
fa07becf... from its own request through our canonical implementation,
proving we hash the same material flex-auth does rather than pinning a
constant we cannot reproduce.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E4tNMAYcSQmZWUE4wqP4ij

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 715726@bnt-lap001
Assistant-Session: 80a42b32-cba6-4b23-8be0-68819b1a6092

src/secrets_engine/approval_claim.py

@@ -150,11 +150,25 @@ def validate_approval_claim(
     # supplies a binding built in that vocabulary (see the note in
     # resolve_consume_binding about the missing mapping).
     if expected_pdp_digest:
+        # GH-DEC-2026-008 / approval-engine schema v3: pdp_path is the issuer's
+        # DECLARATION that this approval was requested against a bound
+        # CheckRequest, and it guarantees pdp_digest is non-null. Path intent is
+        # never inferred from a pdp_digest that merely happens to be present --
+        # a digest recorded for some other reason is not a declaration anybody
+        # made, and approvals issued before schema v3 carry pdp_path false
+        # regardless of any digest they hold.
+        if binding.get("pdp_path") is not True:
+            raise DecisionError(
+                "approval claim does not declare binding.pdp_path; it was not "
+                "issued against a bound CheckRequest and cannot authorize this "
+                "action (GH-DEC-2026-008). Request an approval bound at issue."
+            )
         if not pdp:
             raise DecisionError(
-                "approval claim records no pdp_digest, and no published mapping "
-                "exists between approval-engine and secrets-engine action/target "
-                "vocabularies; the claim cannot be tied to this exact action"
+                "approval claim declares pdp_path but records no pdp_digest, and "
+                "no published mapping exists between approval-engine and "
+                "secrets-engine action/target vocabularies; the claim cannot be "
+                "tied to this exact action"
             )
         if pdp != expected_pdp_digest:
             raise DecisionError("approval claim pdp digest does not match the request")