State tenant-engine's tenant relation in the write-api package, v3.

tenant-engine named the relation in TEN-DEC-2026-002: `tenant` is the target
tenant record and always equals `resource.id`; the write API is cross-tenant
by design and `tenant.guardrail.read` does not differ. v2 carried no tenant
rule and a constant fixture tenant, so the deliberate scope and an omitted
rule were indistinguishable.

tenant-engine.write-api.mutate v3 (FLEX-DEC-2026-016):
- allowed requires tenant_is_target; a mismatch or absent tenant is denied
  tenant_not_target (object.get, so an absent key names the right cause).
- the cross-tenant scope is stated in the package and quantified by
  test_tenant_never_changes_effect over every action, three subjects and
  four tenants, with guards against passing by denying everything.
- fixtures rotate tenant across four tenants; five cross-tenant allows and
  two tenant_not_target denies added (42 fixtures, 33 tests, all pass).
- user-engine's tenant:platform exclusion is named as a fixed-record rule,
  not a subject/tenant relation, and tested separately.

Closes FLEX-WP-0022 (T01, T02 done). Also records TEN-IN-0004 and
SECRETS-IN-0002 on FLEX-WP-0020 and acknowledges the GH-DEC-2026-017
replies on FLEX-WP-0030-T04.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 63291@bnt-lap001
Assistant-Session: 8bd77868-ca68-4f49-bb1e-d539ecc0d703

workplans/FLEX-WP-0030-boundary-declaration-cleanup.md

@@ -275,6 +275,17 @@ A13 notes it. B2: `gate-house` declared; `key-cape`, `ops-mason` and
 `net-kingdom` have not answered. Task stays `progress` on B2 alone — not closed
 by silence. Consequences are T06–T08.
 
+2026-09-21 (acknowledgements): `kings-guard`'s reply to the B1 correction
+(`4c328bf6`) predates `GH-DEC-2026-017`; it held its two forms rather than
+guessing, recorded `KG-IN-0007`, and has since applied the ruling and closed
+that intake (`kings-guard` `5120adf`, value unchanged, `standard_version`
+removed). Its observation — that off-vocabulary values are a third class a
+precedence ruling alone leaves untouched — is answered by the post-ruling
+survey (`docs/evidence/2026-09-21-layer-declaration-survey-after-ghdec017.json`):
+`off_vocab` is empty once `Taxonomy` is admitted. `approval-engine`,
+`maturity-engine`, `zone-engine`, `secrets-engine` and `railiance-master` also
+confirmed the ruling applied. None of these bears on B2.
+
 ## Out of scope
 
 - Bumping flex-auth to declare v0.8. v0.8 is `status: proposed`; T01 removes the