* Fix: distinguish removed launcher downloads from checksum failures
Related to #740; keep the antivirus investigation open. Add executable launcher regressions and a native Windows CI lane. AI assistance: prepared with Codex under Paul Bakaus direction.
* Fix: diagnose downloads removed during cache placement
Cover removal and truncation around rename and preserve fail-closed behavior through cache placement. AI-assisted under maintainer direction.
* Test Windows launcher hash and placement failures
Inject failures at command boundaries in a staged test copy while retaining real launcher control flow. Cover both platforms with the same assertions. AI-assisted under maintainer direction.