The 3.10 attempt did not just fail — it left /AI/guffcache/longcat_venv-train
behind, pyvenv.cfg saying version = 3.10.18. ensure_train_built skips creation
when the directory exists, so the next run would have pip-installed into that same
3.10 and failed identically, forever. Fixing the interpreter choice in v0.2.56 was
not enough on its own for anyone who had already tried once.

An existing training venv is now checked against what SimpleTuner needs
(>=3.12,<3.15) and rebuilt when it does not qualify, saying so in the log. The
version is READ from pyvenv.cfg rather than obtained by running the interpreter:
the Python that built it may be gone, and a failed exec is indistinguishable from
a venv that is merely incomplete.

Found by looking at the directory before telling the operator to press the button
again, which is the only reason it is in this commit rather than in their next log.

24 tests.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY