docs: add TOPOLOGY.md

Author: hyperpolymath

TOPOLOGY.md

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+<!-- SPDX-License-Identifier: PMPL-1.0-or-later -->
+<!-- Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk> -->
+# TOPOLOGY.md — a2ml-validate-action
+
+## Purpose
+
+GitHub Action that validates `.a2ml` manifest files in a repository. Checks SPDX headers, required identity/version fields, attestation block structure, and section syntax. Used in CI pipelines across RSR repos to enforce A2ML compliance.
+
+## Module Map
+
+```
+a2ml-validate-action/
+├── action.yml            # GitHub Action metadata and entry point
+├── src/
+│   └── validate-a2ml.sh  # Validation script (delegates to shell logic)
+├── examples/             # Example workflows using this action
+├── docs/                 # Usage documentation
+└── container/            # Containerfile for CI
+```
+
+## Data Flow
+
+```
+[GitHub workflow trigger] ──► [action.yml] ──► [validate-a2ml.sh]
+                                                       │
+                                            [scan repo for .a2ml files]
+                                                       │
+                                               [pass / fail with annotations]
+```