Modernize packaging and continuous integration

[waddell]

Summary

• replace legacy setup.py metadata with a modern pyproject.toml
• require Python 3.10 or newer and test Python 3.10 through 3.14
• consolidate five stale workflows into one least-privilege CI workflow
• test Linux, macOS, and Windows and validate built package artifacts
• restore compatibility with current pandas and NumPy releases
• preserve the v1.8 write_tables functionality already merged into main

Validation

• 74 tests passed on Python 3.12 with pandas 3.0 and NumPy 2.5
• 96% source coverage
• Ruff checks passed
• source distribution and wheel passed strict metadata checks
• wheel installed and imported successfully in a clean environment
• all eight GitHub Actions jobs passed

Refs #63.