fix(nix): use python312 for package env

[OwenYWT]

Summary

• switch the uv2nix package environment from python311 to python312
• update the dev shell to create Python 3.12 virtual environments
• add static regression tests for the Nix Python version wiring

Root cause

The current nixpkgs pin includes Sphinx 9.1, which no longer supports Python 3.11. The Nix package builder hardcoded python311, so flake evaluation could fail when pyproject-build-systems wired the Sphinx hook into the Python 3.11 package set.

Test plan

• python -m pytest -o addopts='' tests/test_nix_metadata.py -q
• python -m py_compile tests/test_nix_metadata.py
• rg -n "python311|Python 3\.11" nix/python.nix nix/devShell.nix (no matches)
• git diff --check

Not run: nix flake check / nix run because nix is not installed in this local environment.

Related: #9526

[RuckVibeCodes]

[gus-first-pass] fix(nix): use python312 for package env - Simple solution, no concerns.

[boozedog]

yes please

i have tested same fix in nix-darwin and it's working for me

[alt-glitch]

Superseded by #12208 (merged) which upgraded Python 3.11 → 3.12 in the Nix package and touches the same nix/python.nix and nix/devShell.nix files.