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/test_approval_claim.py

@@ -122,6 +122,7 @@ def test_principal_falls_back_to_actor_when_absent():
 def test_pdp_digest_is_preferred_when_the_issuer_recorded_one():
     claim = _claim()
     claim["binding"]["pdp_digest"] = "sha256:" + "a" * 64
+    claim["binding"]["pdp_path"] = True
     claim["binding"]["digest"] = "sha256:" + "b" * 64  # native no longer matches
     assert _validate(claim, expected_pdp_digest="sha256:" + "a" * 64)