Review finding: with the install's own venv activated (a re-run), `uv python
find <range>` returned venv/bin/python3, which setup_venv then deleted before
handing the dead path to `uv venv`; the stage still exited 0 and printed
"Virtual environment ready". Probe with --system so only base interpreters
qualify, and fail the stage when the venv was not created.