fix(chroma): write blob-fix marker even when narrowing skips all rows

The narrowed _fix_blob_seq_ids returned early when safe_rows was empty,
but #1177's marker contract requires the marker to be written on every
successful pass — even no-op — so subsequent opens skip the sqlite3
connection entirely. Without this, palaces that have no genuine 0.6.x
BLOBs but DO have sysdb-10-prefixed rows would re-open sqlite3 on every
call, defeating the #1090 corruption guard.

Restructured the conditional so the marker write is unconditional after
a successful sqlite scan, regardless of whether any rows were updated.

Surfaced by test_fix_blob_seq_ids_writes_marker_when_already_integer
during the develop-rebase of this PR. The author's branch predates the
marker contract from #1177 (merged 2026-04-26), so this is a rebase-edge
fix-up rather than a logic change to their narrowing behaviour.

tests/test_backends.py

@@ -507,6 +507,7 @@ def test_fix_blob_seq_ids_skips_sqlite_when_marker_present(tmp_path):
 
     mock_connect.assert_not_called()
 
+
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