Add notebook anywidget support via D3ARG.to_anywidget()

[Billyzhang1229]

Summary

This PR adds D3ARG.to_anywidget(...) to render ARG visualizations as an anywidget object in notebook environments (e.g. marimo).

What changed

• Added to_anywidget() to D3ARG in tskit_arg_visualizer/init.py.
• Reused existing visualizer.js + visualizer.css rendering path by generating srcdoc for an iframe-based anywidget.
• Added anywidget dependency in:
  • requirements.txt
  • environment.yml
  • pyproject.toml
• Updated Python metadata for dependency compatibility:
  • pyproject.toml: requires-python = ">=3.8"
  • environment.yml: python>=3.8
  • Classifiers now include 3.8 through 3.13

marimo rendering fix

• Added base iframe styles (margin:0, overflow:hidden, box-sizing:border-box) to avoid inner overflow and duplicate-looking horizontal scrollbars.

Validation

• Internal smoke test: created D3ARG from msprime output and called to_anywidget(...).
• Verified widget object is returned with non-empty srcdoc and expected frame dimensions.