[Montana]

GitHub retires the macos-14 runner image on 2026-11-02. Until then it cancels every macos-14 job during scheduled brownouts (actions/runner-images#13518):

• 14:00 to 24:00 UTC on Oct 29, 30 and 31 and Nov 1;
• after Nov 2, every macos-14 job fails, on every PR.

Today #1412's three macOS legs were cancelled after 1 s each, twice:

• Python suite · macos-14 · 1/2 and 2/2;
• MIR codegen · macos-14.

The annotation reads "This job failed because of a scheduled macOS 14 runner image brownout."

Change

macos-14 → macos-15 (arm64) in every runner list:

file	job
.github/workflows/python-tests.yml	the Python suite leg, its label, and the counts job's label list
.github/workflows/mir.yml	MIR codegen
.github/workflows/wheels.yml	arm64 wheels
.github/workflows/release.yml	arm64 release wheels

It mirrors 2ed5c4d, which moved the Intel legs from the retired macos-13 to macos-15-intel.

• Wheels: they keep MACOSX_DEPLOYMENT_TARGET: "11.0", so the arm64 wheel still targets macOS 11; only the build host changes.
• Text references: native-tests.yml's comment, SUPPORT_MATRIX.md (the wheel-matrix row and the Python-gate sentence) and python/tests/conftest.py's comment describe the current CI, and are updated. Historical mentions (CHANGELOG, test comments about past macos-14 runs) are left as they were.

For a maintainer

• Required checks: the check names change, from Python suite · macos-14 · 1/2 to · macos-15 · 1/2, and the same for MIR codegen and Python suite counts. If branch protection lists the old names as required, they need updating, or every PR will wait on checks that never run. I can't read the protection settings from the fork (404).
• Build cache: the leg's cache key moves (python-tests-macos-15-build), so its first run builds cold.

Not verified

This PR's own CI is the first run on macos-15. wheels.yml and release.yml run only on their own triggers, so their arm64 legs aren't exercised here.