fix(regression): separate recipe for bad-restore (custom-html-rst-bad)

Having test_backup.py in custom-html-bkp-bad caused both bad-backup and bad-restore
to fail at the backup tier. Create custom-html-rst-bad with its own cc-ci test dir
that has ops.py+test_restore.py but NO test_backup.py, so:
- backup: only generic test_backup_artifact → PASS (snapshot exists)
- restore: pre_restore writes 'mutated', marker stays 'mutated' after restore → FAIL

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/regression/test_canaries.py

@@ -141,13 +141,13 @@ _BAD_BACKUP = {
 
 _BAD_RESTORE = {
     "id": "bad-restore",
-    "recipe": "custom-html-bkp-bad",
-    "src": "recipe-maintainers/custom-html-bkp-bad",
-    # Pin: custom-html-bkp-bad main @ 2026-06-02 (b6fe99de).
-    # No pre_backup hook → marker never seeded before backup → snapshot has no ci-marker.txt.
-    # pre_restore writes "mutated". After restore, marker is MISSING or "mutated" → FAIL → restore=RED.
-    # install+backup PASS; upgrade=skip (no version tags yet).
-    "ref": "b6fe99de41601f9e51bc7ea5b6072f0c3f56cdc3",
+    "recipe": "custom-html-rst-bad",
+    "src": "recipe-maintainers/custom-html-rst-bad",
+    # Pin: custom-html-rst-bad main @ 2026-06-02 (9a73a184).
+    # No pre_backup hook → backup snapshot has no ci-marker.txt.
+    # pre_restore writes "mutated". After restore: marker stays "mutated" → FAIL → restore=RED.
+    # install+backup PASS (no test_backup.py in cc-ci dir); upgrade=skip (no version tags).
+    "ref": "9a73a184e739691bc6a621a5f1e6efc799743c5b",
     "expected_green": False,
     "stages": "install,backup,restore,custom",
     "failing_tier": "restore",