feat(regression): add E2E canary regression suite (tests/regression/)

Seven canaries prove both halves of the server's job:
- GREEN: good apps are reported healthy (good-simple + good-significant)
- RED: broken apps are caught at intended tier (false-green guard + 4 per-tier)

Fixtures: custom-html-bkp-bad (backup tier RED) + custom-html-rst-bad (restore tier RED).
All 7 canaries verified on live server (see STATUS-regression.md for artifacts).

Not wired to per-commit CI — run on-demand: pytest -m canary tests/regression/

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

tests/custom-html-rst-bad/ops.py

@@ -0,0 +1,15 @@
+"""custom-html-rst-bad — lifecycle ops for bad-restore RED canary.
+
+NO pre_backup hook: marker never seeded before backup → snapshot has no ci-marker.txt.
+pre_restore writes "mutated". After restore, marker stays "mutated" (not in snapshot) → FAIL.
+"""
+
+from __future__ import annotations
+
+from harness import lifecycle
+
+MARKER_PATH = "/usr/share/nginx/html/ci-marker.txt"
+
+
+def pre_restore(domain: str, meta: dict) -> None:
+    lifecycle.exec_in_app(domain, ["sh", "-c", f"echo mutated > {MARKER_PATH}"])

tests/custom-html-rst-bad/recipe_meta.py

@@ -0,0 +1,3 @@
+# custom-html-rst-bad — regression fixture for bad-restore canary.
+# BACKUP_CAPABLE=True forces the backup tier to run even though the recipe has no backupbot label.
+BACKUP_CAPABLE = True

tests/custom-html-rst-bad/test_restore.py

@@ -0,0 +1,23 @@
+"""custom-html-rst-bad — RESTORE assertion (bad-restore RED canary).
+
+No pre_backup → backup snapshot has no ci-marker.txt. pre_restore writes "mutated".
+After restore: marker is "mutated" (restore can't recover "original" — wasn't backed up) → FAIL.
+"""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from harness import lifecycle  # noqa: E402
+
+MARKER_PATH = "/usr/share/nginx/html/ci-marker.txt"
+
+
+def test_restore_returns_state(live_app):
+    result = lifecycle.exec_in_app(
+        live_app, ["sh", "-c", f"cat {MARKER_PATH} 2>/dev/null || echo MISSING"]
+    ).strip()
+    assert result == "original", (
+        f"restore did not return the pre-mutation (backed-up) state: got {result!r}. "
+        "Expected 'original'. The backup had no marker, so restore cannot recover it."
+    )