feat: complete and prove the authorization chain end to end

Implements step 2 (access-engine POST /v1/check) and wires the whole
GH-DEC-2026-003 sequence together, then proves it against a live throwaway
OpenBao rather than only unit-level fakes.

- decision_check.check_decision performs the PDP call; an unreachable or
  non-200 PDP raises, since silence is never permission.
- approval_consume.authorize_action coordinates steps 1 and 2 and returns an
  AuthorizedAction. Both steps build the same CheckRequest via a shared
  _expected_request, since two descriptions of the action cannot produce
  corresponding digests.
- apply_unreachable_engine_stance takes authorized=. The published map
  defines fail_closed as no side effect WITHOUT a durable decision record,
  so holding a validated one means the residue does not apply. Not a bypass:
  both steps must have succeeded and CAS consume still precedes OpenBao.
  Unconfigured still returns None and fails closed.

The end-to-end test caught one more instance of the cross-vocabulary bug: a
leftover comparison of the claim's binding.action against ours. The claim
says secrets.kv.destroy where we say destroy, so it would have failed against
every real claim. Removed; the tie is pdp_digest.

Integration coverage asserts PIP-then-PDP ordering, that consume is the last
step before the backend, and that an unreachable PDP, denied decision,
invalid claim, missing pdp_digest, consume conflict and action mismatch each
stop before OpenBao. 284 tests pass; production still fails closed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M65ovP3eiiPHubibvWs9mD

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 393550@bnt-lap001
Assistant-Session: 4bb359f9-1f12-4410-9e76-079cf23c82e4

docs/approval-consumption.md

@@ -111,6 +111,7 @@ did before. Production additionally needs:
 | `SECRETS_ENGINE_APPROVAL_TOKEN_FILE` | mode-0600 credential, outside Git |
 | `SECRETS_ENGINE_AUTHORIZATION_SUBJECT_ID` / `_SUBJECT_TYPE` | the acting principal |
 | `SECRETS_ENGINE_AUTHORIZATION_POLICY_PACKAGE` / `_VERSION` | the live pin (step 2) |
+| `SECRETS_ENGINE_PDP_URL` / `_PDP_TOKEN_FILE` | the per-consumer access-engine pin |
 
 The distinct-approver threshold is no longer a consumer-side check. The claim
 does not expose approver entries; approval-engine folds that requirement into