ci: use setup-python interpreter

[JigSawPT]

Root cause

The Windows py launcher selected the runner's global Python 3.14 instead of the version provisioned by setup-python. This also installed PyInstaller into a Scripts directory outside PATH, so the tagged release failed before producing any asset.

Fix

• use python -m pip/pytest/compileall in CI so every matrix job uses its configured interpreter
• use python -m PyInstaller in releases to avoid Scripts/PATH ambiguity

Validation

• 15 tests passed with the project interpreter
• compileall passed
• python -m PyInstaller produced the one-file executable locally