longcat: the training venv needs 3.12+, not the inference venv's 3.10 (v0.2.56)

The new diagnostics earned their keep immediately. Last run said only "returned
non-zero exit status 1"; this one said:

  ERROR: Could not find a version that satisfies the requirement simpletuner
         (from versions: none)
  ERROR: No matching distribution found for simpletuner

which reads like the package does not exist. It does. The line above it is the
real one: "Ignored the following versions that require a different python
version: … Requires-Python >=3.12,<3.14". SimpleTuner 4.9.3 needs >=3.12,<3.15,
pip was being run under 3.10, so every release was filtered out and the remainder
was none.

My error. The 3.10 pin belongs to the INFERENCE venv — torch 2.6+cu124,
transformers 4.41, numpy 1.26 — and I applied it to the training venv, which wants
the opposite. They are separate venvs precisely because their requirements
conflict; sharing an interpreter finder was the bug.

ensure_train_built now picks an interpreter SimpleTuner can install into: a
configured longcat.train_python, else this process's own (3.13, which satisfies
>=3.12,<3.15), else python3.13/3.12 on PATH. So the training venv — unlike the
inference one — needs nothing bundled and nothing provisioned. The manual
instructions in the non-auto branch said python3.10 too; they now say 3.12+.

Also renamed _py310_env to _clean_py_env, since it now guards a 3.13 child as well
as the 3.10 one.

The venv itself WAS created this time, so the earlier "No module named
'encodings'" did not recur — whether the stripped environment fixed it or it was
transient, I still cannot say.

17 tests, including that ensure_train_built never reaches for the 3.10 finder.

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