chore: add root Mustfile (RSR-mandatory checks contract)

Adds the root Mustfile required by REQUIRED-FILES.adoc (was missing across
the -iser family). Declares the mandatory checks, each mapping to an existing
Justfile recipe (just lint / test / fmt). Part of the base-camp RSR
compliance sweep.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mbq6yKF9RhFai6EQ7WqKhQ

Author: claude

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