Add first-wave State Hub arc42 stub

Records the read-model and retirement stance. Chapter 9 points at the
estate ADRs that still bind this service.

docs/architecture/state-hub_v0.1.md

@@ -0,0 +1,152 @@
+---
+id: state-hub-architecture
+title: "State Hub architecture"
+status: proposed
+owner: state-hub
+revision: "draft-1"
+version: "0.1"
+last_reviewed: "2026-08-18"
+review_interval: 6m
+---
+
+# State Hub — Software Architecture Documentation (arc42)
+
+## About this document
+
+First-wave arc42 for State Hub, the estate's live coordination
+read-model. This service is in active retirement planning; new
+permanent ownership should not land here. Chapter 9 points at the
+estate ADRs that still bind it.
+
+---
+
+## 1. Introduction and Goals
+
+State Hub is a queryable, auditable memory of work: domains, repos,
+workplans, tasks, decisions, progress. Files remain the origin. The
+hub is derived state (custodian ADR-001, ADR-003).
+
+It remains operational until retirement gates in
+`prj-state-hub-retirement` are met. Replacement ownership is moving
+toward `repo-manager` and `hub-core`.
+
+### 1.1 Requirements Overview
+
+- Rebuild coordination state from registered repository files.
+- One identifier registrar (ADR-007). This workstation is not it.
+- Preserve compatibility; do not take new permanent architectural
+  ownership.
+
+### 1.2 Quality Goals
+
+1. Rebuildability from git.
+2. Hub never becomes the origin of work.
+3. Extraction paths stay open.
+
+### 1.3 Stakeholders
+
+| Role | Concern |
+| --- | --- |
+| state-hub | Live read-model during retirement. |
+| the-custodian | Estate rules the hub must not invert. |
+| repo-manager | Incoming consistency / repo representation. |
+| product repos | Workplan files the hub indexes. |
+
+---
+
+## 2. Architecture Constraints
+
+N/A for this stub — retirement program is the binding constraint.
+
+---
+
+## 3. System Scope and Context
+
+**In:** indexing workplans/tasks/decisions, consistency rebuild,
+query API and dashboard used today.
+**Out:** being the source of work items; new cross-domain capabilities;
+publication of policy (policy-nexus).
+
+### 3.1 Business Context
+
+Files are excellent for canon and provenance. The estate still needs a
+live query surface while retirement proceeds.
+
+### 3.2 Technical Context
+
+Inputs: workplan markdown via `fix-consistency`. Outputs: HTTP/MCP
+APIs. Neighbours: every registered repo, activity-core (ops runs),
+policy-nexus (does not index the hub).
+
+---
+
+## 4. Solution Strategy
+
+N/A for this stub. The strategy is already in the estate ADRs: files
+first, materialized derived state, single registrar, local cache vs
+authority (ADR-010, proposed).
+
+---
+
+## 5. Building Block View
+
+### 5.1 Level 1 – System/Top-Level
+
+N/A for this stub.
+
+---
+
+## 6. Runtime View
+
+N/A for this stub.
+
+---
+
+## 7. Deployment View
+
+N/A for this stub.
+
+---
+
+## 8. Cross-Cutting Concepts
+
+N/A for this stub.
+
+---
+
+## 9. Architecture Decisions
+
+This repo has no `docs/adr/` corpus. Binding decisions live in
+the-custodian and are listed on the estate map:
+
+| Estate ADR | Status | Why it binds this system |
+| --- | --- | --- |
+| CUST-ADR-001 (workplans as repo artefacts) | accepted | Hub is a read model. |
+| CUST-ADR-003 (materialized derived state) | accepted | How the cache invalidates. |
+| CUST-ADR-007 (identity and registrar) | accepted | One writer of workplan UUIDs. |
+| CUST-ADR-010 (hub authority / local cache) | proposed | Two kinds of hub data. |
+
+Do not treat a State Hub `/decisions` row as the published ADR.
+
+---
+
+## 10. Quality Requirements
+
+N/A for this stub.
+
+---
+
+## 11. Risks and Technical Debt
+
+N/A for this stub. Residual: this workstation cannot mint registrar
+UUIDs.
+
+---
+
+## 12. Glossary
+
+| Term | Meaning |
+| --- | --- |
+| Read model | Derived index; never the origin. |
+| Registrar | The single instance allowed to mint workplan UUIDs. |
+| Retirement | Coordinated move of capabilities out of this repo. |