fix: keep the test suite offline everywhere

tests/__init__.py

@@ -0,0 +1,13 @@
+"""No test downloads a release asset unless it says so.
+
+The CI job sets RETROBIOS_OFFLINE; a local run of the documented command
+did not, and e2e fixtures queried the large-files release and wrote its
+answers into the shared .cache/large. A test that serves its own assets
+clears the variable for itself.
+"""
+
+from __future__ import annotations
+
+import os
+
+os.environ.setdefault("RETROBIOS_OFFLINE", "1")