CI: trim build workflow matrix and collapse build+test

[Copilot]

The Build and Test with Mamba workflow ran a full os × python-version matrix (2 OSes × 4 Python versions) across two sequential jobs, producing 16 runs per push/PR with little added signal — the project has no version-specific code paths beyond the >=3.10 floor, and the build job was redundant with test.

Changes to .github/workflows/build.yaml

• Single test job. Dropped the separate build job; uv pip install ".[dev]" in test already exercises the install path. Also removed the actions/cache block that pointed at the wrong env path and never hit — setup-micromamba's cache-environment: true handles this.
• Focused matrix. ubuntu-latest × {3.10, 3.12} (pinned floor + one current Python) plus a single macos-latest × 3.10 smoke test via include:. 16 runs → 3.
• Proper Python override. Replaced the sed rewrite of environment.yaml with create-args: python=${{ matrix.python-version }}.
• Housekeeping. Bumped actions/checkout to v4, added concurrency to cancel superseded PR runs, set fail-fast: false, and added a minimal permissions: contents: read block.

Test invocation is unchanged (pytest -v tests --disable-warnings); environment.yaml's python=3.10 pin is left as-is to document the supported floor.

strategy:
  fail-fast: false
  matrix:
    os: [ubuntu-latest]
    python-version: ['3.10', '3.12']
    include:
      - os: macos-latest
        python-version: '3.10'

Open questions for reviewers: whether to keep the macOS smoke test on every PR or move it to a scheduled/release-only run, and whether 3.13 should be added back once pinned scientific deps officially support it.