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

tests/fixtures/flex-auth-replay/decision_destroy_dual_control.json

@@ -1,5 +1,5 @@
 {
-  "id": "decision:395efe37c5066e8a",
+  "id": "decision:44a40c339a020772",
   "contract_version": "flex-auth.decision-record.v1",
   "request_id": "check:secrets-engine-destroy",
   "effect": "allow",
@@ -72,7 +72,8 @@
           "action": "secrets.kv.destroy",
           "actor": "agt-secrets-engine",
           "digest": "sha256:3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f3f",
-          "pdp_digest": "sha256:570d112890586d3cbf00c0e81c85ae7806f40f00afa1a2c0a23fd5e077a27f56",
+          "pdp_digest": "sha256:fa07becfaa471394d06aee5fa3cd66352bf0cc69ef24900240489684cda8cd56",
+          "pdp_path": true,
           "principal": "bernd",
           "purpose": "rotate-exposed-key",
           "target": {
@@ -98,13 +99,14 @@
         }
       }
     },
-    "request_digest": "sha256:fc155dba88f8ab18b3032ed086ba2f455158c6981106e7829d520ab7b036bdf3"
+    "request_digest": "sha256:c749ee2dc3cdf927a70a3e5b27cff4d97a438d3264153b4b2e3bcacbaf82091a",
+    "approval_binding_digest": "sha256:fa07becfaa471394d06aee5fa3cd66352bf0cc69ef24900240489684cda8cd56"
   },
   "lifetime": {
     "kind": "ttl",
     "ttl": "15m",
-    "not_before": "2026-09-06T12:18:21Z",
-    "expires_at": "2026-09-06T12:33:21Z"
+    "not_before": "2026-09-06T12:51:16Z",
+    "expires_at": "2026-09-06T13:06:16Z"
   },
   "diagnostics": {
     "action": "destroy",
@@ -122,9 +124,9 @@
     "policy_package_digest": "sha256:fe0070b79f66442ae6c218697a49c470c6c8f670aa57a30c078a5284d097bd8c",
     "registry_snapshot_digest": "sha256:f5a309bc0b36721fd6d9ad7f53eb21222162bc2eac62a0ab0802a9a1d51340bb",
     "input_claim_digests": {
-      "context": "sha256:b0d2203cd2b43a9ba573c21c038154c131afba4255e313affd7ecf810e2cc221"
+      "context": "sha256:8b73d29ecef286d42e03d2420531d6c45219f325a7ae004c1ecfc781203a2800"
     },
-    "decision_time": "2026-09-06T12:18:21Z"
+    "decision_time": "2026-09-06T12:51:16Z"
   },
   "caring": {
     "profile": "caring-0.4.0-rc2",