docs(adr-0099): I3 names EXTERNAL as the one posture not derived from capability grants

EXTERNAL derives from the reserved org_external position, which the
resolver projects from sys_member.role = external. I3 said every posture
derives only from capability grants; a dated note records the exception,
and ADR-0095 D3's bullet points to it.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AZbN6PU5Xz4WTi6NKmyKhN

Author: claude

docs/adr/0095-authz-kernel-tenant-layer-and-posture-ladder.md

@@ -271,7 +271,7 @@ enum AuthzPosture {
   source of a position; enforcement reads the posture, never the role. It is
   the one rung whose evidence is an identity position rather than a
   capability bit, and it outranks `TENANT_ADMIN` (only `PLATFORM_ADMIN`
-  precedes it).
+  precedes it). ADR-0099 I3 records the exception.
 - **No enforcement-time code path may consult the better-auth role directly.**
   This removes dual-track adjudication: #2836-class conflicts (explicit deny
   from one track out-composing a wildcard allow from the other) become

docs/adr/0099-posture-adjudicated-tiering-and-external-rung.md

@@ -174,6 +174,11 @@ Invariants (each carries a matrix cell):
 - **I3** — Posture is derived only by the resolver, only from capability
   grants (ADR-0095 D3); no enforcement site re-derives tier at the point of
   decision (fallback probe excepted, per D1, and it may only narrow).
+  _Amended 2026-10-23 (ruling of record 6355206139 on #25233, letter B):
+  `EXTERNAL` is the one exception to "only from capability grants". It
+  derives from the reserved `org_external` position, which the resolver
+  projects from `sys_member.role = external` (ADR-0095 D3); the resolver
+  stays the only deriver, and no enforcement site reads the role._
 - **I4** — Enforcement and explain read the same carried value; a divergence
   is a defect, not a display issue.
 - **I5** — Posture selects tier, never side: no rung grants a read or write