test: the backend mutant follows the registry convention (#329 r2-H1)

The reviewer is right twice over. My previous commit fixed the red CI by
relaxing the contract — a guard could name a `.py` file — when the registry
already had a convention for exactly this case: every backend mutant runs
`node scripts/backend-test-guard.mjs <pattern> <file>`, which owns the python
executable choice and the `-k` selection. Bending a rule to fit my one-off is
the worse of the two possible fixes, so the contract goes back to demanding a
`.mjs` guard, and junction-limit-backend-raw-baseline now uses the helper and
targets the one test that proves the migration
(test_legacy_baseline_is_judged_after_the_same_migration).

Re-verified: registry --check clean, the mutant still catches its regression
1/1, npm test 1390 passed / 0 failed.

Issue: #329
User-Visible: no

scripts/mutation-gate.mjs

@@ -539,7 +539,9 @@ export const MUTANTS = [
   },
   {
     id: 'junction-limit-backend-raw-baseline',
-    guard: 'python3 -m pytest tests_backend/test_junction_limits.py -q',
+    guard: 'node scripts/backend-test-guard.mjs '
+      + 'test_legacy_baseline_is_judged_after_the_same_migration '
+      + 'tests_backend/test_junction_limits.py',
     because: 'a legacy baseline carries no wall catalogue, so judging it raw reports "no '
       + 'violations" whatever its geometry and turns every inherited one into a refusal of '
       + 'an unrelated edit (#329 §3, code review r1 H1)',