feat(2): bluesky-pds P4 data-integrity overlay — deterministic atproto account marker (recipe-aware; catches running-app-holds-sqlite restore gap) via _p4.py + ops/test_upgrade/backup/restore

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/bluesky-pds/test_backup.py

@@ -0,0 +1,13 @@
+"""bluesky-pds — BACKUP overlay (Phase 1e HC3): assertion-only + additive.
+ops.pre_backup created the deterministic marker account before the backup op; this overlay asserts it
+is present at backup time (so the backup archive captures it)."""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(__file__))
+import _p4  # noqa: E402
+
+
+def test_backup_captures_state(live_app):
+    assert _p4.account_exists(live_app), "seeded marker account not present at backup time"