fix(install): require python3 3.11+ in preflight

[alexwbend]

What & why

The installer's preflight checks that git and python3 exist, but not which Python. The engine reads workflow.toml with tomllib, stdlib only from 3.11 — so this adds the version to the same preflight block, in the same have … || die style.

On Ubuntu 22.04 LTS (ships 3.10) or macOS system Python (3.9) the installer today reports success:

✓ TRON 0.4.3 installed.

and the first tron start then dies far from the cause, inside a module the user has never opened:

File ".../engine/workflow.py", line 18, in <module>
    import tomllib
ModuleNotFoundError: No module named 'tomllib'

After, it fails at the gate with the fix named, before anything is cloned or symlinked:

✗ python3 is 3.10 — TRON needs 3.11 or newer (the engine reads workflow.toml
with tomllib). Install Python 3.11+ and re-run.

The minimum isn't recorded anywhere today — Getting Started's Requirements says "python3 and git" — so the header comment now carries it next to the check that enforces it.

Notes:

• Placed inside the existing preflight block so the ordering rule holds: refuse before touching the filesystem. Nothing is cloned or linked on the failure path.
• PY_VER falls back to ? if python3 -c cannot run at all, so the message is never blank.
• The || die form is safe under set -eu — POSIX ignores -e for any command of an AND-OR list other than the last, so a failing probe routes to die rather than exiting mute.
• Behaviour on a supported Python is unchanged.

Source of truth touched?

• engine/workflow.toml → python3 engine/workflow.py --write (docs/WORKFLOW.md) + python3 engine/bpmn.py --write (workflow/)
• engine/glossary.py → --write (docs/GLOSSARY.md)
• engine/events.py → --write (docs/EVENTS.md)
• N/A — no generated artifact affected

Checklist

• Full selftest suite passes locally (engine/*.py, evaluation/harness.py --selftest)
• Least-necessary change; matches the surrounding code
• Docs/comments updated where behaviour changed

Verified both arms on this branch: sh -n install.sh clean; the gate fires on a real 3.10 and exits 1 before cloning; with a python3 reporting 3.12 the install runs through to ✓ TRON 0.4.3 installed. unchanged. No selftest or CI job reads install.sh, and the file mode stays 100755.