add resolution option to matrix to test minimum stated deps (opt in)

[tlambert03]

this PR

• updates used actions to the latest versions
• makes py 3.10 the default (3.9 is EOL next week) ... though 3.9 is still in the options
• adds an actionlint test to make sure the rendered workflows are valid
• most significant change: adds a new resolution: ["highest", "lowest-direct"] to the matrix. This (uv-dependent feature) is super useful to make sure that your full dependency constraints work. Currently off by default, (must be selected in customize) since it requires '>=' for all your pins in pyproject

this ends up looking like this:

  test:
    name: ${{ matrix.platform }} (${{ matrix.python-version }}) [${{ matrix.resolution }}]
    runs-on: ${{ matrix.platform }}
    env:
      UV_PRERELEASE: ${{ github.event_name == 'schedule' && 'allow' || 'if-necessary-or-explicit' }}
      UV_RESOLUTION: ${{ matrix.resolution }}
      PYTEST_ADDOPTS: ${{ matrix.resolution == 'lowest-direct' && '-W ignore' || '' }}
    strategy:
      fail-fast: false
      matrix:
        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
        platform: [ubuntu-latest, macos-latest, windows-latest]
        resolution: ["highest", "lowest-direct"]