Base camp: add root Mustfile (RSR compliance) (#157)

Part of the estate-wide **base camp** sweep across the -iser family
(coordination atlas + generator fix in iseriser#60).

- **`Mustfile`** (root) — RSR-mandatory checks contract (was missing
family-wide); maps to existing `just lint`/`test`/`fmt`.

This repo was already ahead on the other base-camp items (no banned
`*.res`; CodeQL matrix is `[rust]` — scanning actual source, left
untouched), so only the Mustfile was needed.

**Flag-only (not changed):** this repo's `.claude/CLAUDE.md` declares
`PMPL-1.0-or-later`, which conflicts with the `standards` five-way
classification (PMPL = exactly 3 repos, not this one). Licence changes
are owner-only/flag-only — surfaced, not touched.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01Mbq6yKF9RhFai6EQ7WqKhQ

---
_Generated by [Claude
Code](https://claude.ai/code/session_01Mbq6yKF9RhFai6EQ7WqKhQ)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

Mustfile

@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: MPL-2.0
+# Mustfile — hyperpolymath mandatory checks
+# See: https://github.com/hyperpolymath/mustfile
+#
+# Declarative contract of checks that MUST pass. Each maps to a recipe
+# that already exists in this repo's Justfile.
+version: 1
+
+checks:
+  - name: security
+    run: just lint
+  - name: tests
+    run: just test
+  - name: format
+    run: just fmt