fix: compare structured binding fields, not a digest we cannot reproduce

The live proof in 03c0569 showed validate_decision_envelope rejecting every
real allow. The evaluator normalizes before hashing -- the request tenant is
copied onto subject and resource, and a registry hit copies type, tenant and
selected attributes onto the refs -- so binding.request_digest covers
material we never sent. Byte-equality against our unenriched request was
unsatisfiable, not merely mismatched.

THE RULE WAS ALREADY PUBLISHED. flex-auth's canonical-request-digest.md
section "Normalization" states the enrichment and tells consumers what to do
instead: compare structured binding fields to the proposed action, treat
request_digest as the evaluator's statement of what it hashed, and recompute
independently over the tuple the binding carries. I raised this with them as
an unpublished gap and asked them to pick between three shapes; it was in
their contract already and the answer was the first of the three. Nothing
was blocked on them, and this follows the published rule rather than one I
inferred.

- _require_binding_corresponds: everything we proposed must survive
  unchanged -- tenant, action, context, subject.id/type,
  resource.id/type/system, and every attribute we sent.
- Enrichment may add only type, tenant, attributes. Any other added field is
  refused, and an enriched tenant must be the request tenant, so a
  cross-tenant binding cannot arrive wearing our request's clothes.
- request_digest is still verified, now against binding_tuple(binding) for
  self-consistency rather than against material we never sent.
- The envelope's top-level subject/resource get the same rule; they are
  enriched too.

Proved against the artifact: the real decision:0f9c98f14545c42d now
validates, and the unrefreshed envelope is refused on lifetime -- reaching
the lifetime check at all is the evidence the binding checks pass on a real
decision. Negatives cover a restated resource.attributes.stage, a foreign
subject.tenant, and an unexpected enrichment field.

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

docs/tenant-alignment.md

@@ -116,9 +116,9 @@ alone.
   port-forward` plus a bounded `TokenRequest` token; see
   `docs/pdp-access-path.md`. Adopted and enforced in
   `decision_check.require_supported_pdp_address`.
-- The evaluator's **enrichment rule**: `binding` carries registry-enriched
-  `subject.attributes`, `subject.tenant` and `resource.tenant` that we did not
-  send, so the digest join fails against a real request. flex-auth's to publish.
+- ~~The evaluator's enrichment rule~~ — **already published** in flex-auth's
+  `canonical-request-digest.md` ("Normalization"), and now implemented. See
+  `docs/pdp-access-path.md`.
 
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