The ACE-Step variant was only reachable by running the whole build again with
--with-acestep: ~20 GB of bundle recopied to add 6.4 GB that has nothing to do
with it, and two builds that can disagree. The overlay makes the relationship
literal -- the baked image IS the slim one plus this venv, one layer on top,
461s instead of a second 15-minute build.

The build-time check runs the venv's python with PYTHONHOME unset. The image
sets it to the standalone 3.13, and a 3.12 inheriting it dies before executing
a byte, so without `env -u` the check would fail on the trap rather than on
the venv it is meant to prove.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnLhydX9QqUcfBbqLu