feat(869-c1): activation-capability probe (Part of #869)

Part of #869

Mos (id-11) Gate-16 merge: independent 3-round APPROVE @c5a2bcc5, author id2 != approver id11, CI green wp1973.

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/mosaic/src/mutator-gate/fail-closed-regression.spec.ts

@@ -0,0 +1,41 @@
+import { spawnSync } from 'node:child_process';
+import { join } from 'node:path';
+
+import { describe, expect, it } from 'vitest';
+
+/**
+ * C-REGRESS (issue #869, Point-1) — proves the fail-closed gate is untouched
+ * by the C1 activation probe added alongside this test.
+ *
+ * `mutator-gate.py`'s fail-closed-on-absent-identity behavior is INTENTIONAL
+ * and TEST-LOCKED: #869 C1 gates the WIRING decision for enforcement (via
+ * `leaseEnforcementActivatable()`), it does not — and must not — touch the
+ * gate's own runtime denial behavior. This spec runs the two test-locked
+ * cases from `runtime_tools_unittest.py` directly (rather than merely
+ * re-asserting the same logic in TypeScript) so a regression in the actual
+ * Python gate is caught here too, not just documented in prose.
+ */
+
+const MUTATOR_GATE_DIR = new URL('.', import.meta.url).pathname;
+const UNITTEST_FILE = join(MUTATOR_GATE_DIR, 'runtime_tools_unittest.py');
+
+const LOCKED_TEST_CASES = [
+  'ExecutableEntrypointTest.test_gate_entrypoint_denies_when_identity_environment_is_absent',
+  'MutatorGateTest.test_environment_generation_and_request_failures_deny',
+] as const;
+
+describe('mutator-gate fail-closed behavior (C-REGRESS, unchanged by #869 C1)', () => {
+  it.each(LOCKED_TEST_CASES)('%s still passes', (testCase) => {
+    const result = spawnSync('python3', ['-m', 'unittest', `${moduleName()}.${testCase}`, '-v'], {
+      cwd: MUTATOR_GATE_DIR,
+      encoding: 'utf-8',
+    });
+
+    expect(result.status, `stderr:\n${result.stderr}`).toBe(0);
+  });
+});
+
+function moduleName(): string {
+  // runtime_tools_unittest.py, addressed as a bare module name for `python3 -m unittest`.
+  return UNITTEST_FILE.split('/').pop()!.replace(/\.py$/, '');
+}