Prep release

[chmp]

Prepare the final marrow = 0.3.0 and serde_arrow = 0.15.0 releases.

• Bump marrow from 0.3.0-rc.1 to 0.3.0
• Bump serde_arrow from 0.15.0-rc.2 to 0.15.0
• Update serde_arrow's marrow dependency to 0.3.0
• Regenerate Cargo.lock
• Finalize the serde_arrow and marrow changelog entries for release

Checklist

• Run the pre-commit checks locally with uv run python x.py precommit
• Add tests for any new functionality
• Update the relevant changelog (Changes.md for serde_arrow, marrow/Changes.md for marrow)