Propose ADR-0017: root-grant custody, tenant:platform mapping, observation evidence

Answers user-engine's USER-WP-0037-T02 request; custody doc points to it.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: sonnet
Assistant-Process: 6969@bnt-lap001
Assistant-Session: e8ae73f2-8385-4c9c-b714-94c3c52cdbc0

docs/adr/ADR-0017-root-grant-custody-and-account-authority.md

@@ -0,0 +1,91 @@
+---
+id: NK-ADR-0017
+type: architecture-decision-record
+title: "The Root Grant Is Derived From the Custody Record, Scoped to tenant:platform, and Observed as Non-Secret Evidence"
+status: proposed
+owner: net-kingdom
+revision: "1"
+decided: "2026-09-29"
+last_reviewed: "2026-09-29"
+review_interval: 12m
+---
+
+# ADR-0017 - Root-Grant Custody and Account Authority
+
+**Status:** Proposed (drafted on the defaults agreed with the operator; accept after user-engine review)
+**Date:** 2026-09-29
+**Deciders:** Bernd Worsch, Claude
+
+## Context
+
+user-engine implemented `account.authority.read` (USER-WP-0037-T01) and a
+workload-authenticated lookup endpoint. It will not derive
+`AccountAuthority.root-grant` from JWT roles, a username, or a consumer's
+configured root subject, and USER-WP-0037-T02 is blocked on NetKingdom
+specifying the authority. `docs/platform-root-custody.md` covers custody of the
+platform-root credential and OpenBao material, but not how a workload-facing
+"subject X holds the root grant" fact is derived, revoked, or evidenced.
+
+## Decision
+
+### 1. Authority: the custody record; directory membership is a projection
+
+The **platform-root custody record** (the non-secret ceremony record: custodian
+assignment, quorum plan, root-token disposition, signoff) is the single
+authority for who holds the root grant.
+
+- A root grant exists for a subject only while a custody record names that
+  `(issuer, subject)` as king/platform-root custodian and no later record
+  revokes it.
+- LLDAP `platform-root` membership and IAM claims (`groups: platform-root`,
+  `roles: platform-root-custodian`) are **projections** of the record. They are
+  authentication facts and MUST NOT be used alone to derive a root grant.
+- **Revocation** is a recorded custody action: a new record entry that
+  supersedes the grant, with actor, reason, and time. Revocation must not
+  remove recovery authority (break-glass under king custody stays intact).
+- If the record and a projection disagree, the record wins and the mismatch is
+  a posture finding, not a grant.
+
+### 2. Mapping: `platform:root` grants `tenant:platform` only
+
+- A root grant is expressed as relation `platform-root-custodian` in tenant
+  context `tenant:platform`. It is scoped to the platform control plane.
+- It is never inherited by, or implied for, any other tenant, and tenant admin
+  authority never implies it (consistent with the user-engine boundary
+  contract and IAM Profile v0.3).
+- The grant is a distinct grant type in the membership envelope
+  (`scope_type: tenant`, `scope_id: platform`, `relation:
+  platform-root-custodian`, `owner_system: net-kingdom-custody`). Consumers
+  such as the Hub read it from user-engine; they do not mint it.
+
+### 3. Evidence: non-secret observations with explicit freshness
+
+An account/entitlement observation retained by a consumer contains only:
+`subject` (`issuer`, `subject`), `grant`, `source` (custody record id and
+revision), `observed_at`, and `revoked_at` or `expires_at` when known. It never
+contains tokens, keys, unseal shares, recovery codes, or reset links.
+
+- Observations are read models: they carry `observed_at` and are re-derived from
+  the custody record rather than treated as durable truth.
+- A consumer MAY cache an observation for at most **15 minutes**; sensitive
+  platform actions (root-grant-dependent writes) MUST re-check and fail closed
+  when the record cannot be read or the observation is stale.
+- Revocation takes effect at the next re-check; the cache bound is the maximum
+  revocation latency.
+
+## Consequences
+
+- user-engine can represent `AccountAuthority.root-grant` deterministically from
+  the custody record id and revision, and unblocks USER-WP-0037-T02.
+- The custody record needs a machine-readable, non-secret form that a workload
+  can read. Its shape and location are an open follow-up (below); until it
+  exists, user-engine returns "no root grant" rather than inferring one.
+- The 15-minute bound and the projection-versus-record rule are decisions of
+  this ADR and may be tightened at review.
+
+## Open follow-ups
+
+- Define the machine-readable custody-record schema and read surface (owner:
+  net-kingdom; delivery of the read path may involve railiance-platform).
+- Confirm with user-engine that the membership-envelope mapping above is
+  sufficient for `AccountAuthority`.