Add 3D mesh visualization with plotters

[nash1111]

Summary

• Add 3d_plot example that generates 5 PNG visualizations using plotters' 3D charting
• Painter's algorithm for depth sorting + diffuse lighting shading
• Add rendered images to README in gallery tables

Visualizations

Image	Algorithm	Description
marching_cubes_sphere.png	Marching Cubes	Sphere isosurface (884 faces)
marching_cubes_torus.png	Marching Cubes	Torus isosurface (1584 faces)
marching_cubes_gyroid.png	Marching Cubes	Gyroid isosurface (17429 faces)
voxel_mesh.png	Voxel Mesh	Sphere predicate, 4x4x4 resolution
octree_mesh.png	Octree	Sphere predicate, depth=3

Test plan

• cargo run --release --example 3d_plot generates all 5 PNGs
• cargo fmt --check clean
• cargo clippy -- -D warnings clean

Closes #50

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new 3D visualization example using Plotters’ 3D charts to render surface triangle meshes to PNGs, and updates the README to display the resulting gallery images.

Changes:

• Add examples/3d_plot.rs to render Marching Cubes, voxel, and octree surface meshes to PNG with simple lighting + painter’s algorithm depth sorting.
• Add generated image asset(s) under examples/ for inclusion in documentation.
• Update README to document the new example command and embed the PNG gallery tables.

Reviewed changes

Copilot reviewed 2 out of 7 changed files in this pull request and generated 2 comments.

File	Description
examples/3d_plot.rs	New Plotters-based 3D rendering example producing multiple PNG visualizations.
examples/voxel_mesh.png	Adds a rendered PNG output used in the README gallery.
README.md	Documents the new 3d_plot example and embeds generated images in tables.

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[Copilot]

sorted_faces.sort_by recomputes face_center_depth (and thus sin/cos) multiple times per comparison, which is unnecessarily expensive for larger meshes (e.g., gyroid). Consider precomputing a depth value per face (or caching yaw/pitch sin/cos) and sorting by that key instead.

[Copilot]

partial_cmp(...).unwrap() can panic if the depth is NaN (or otherwise not comparable). Prefer f64::total_cmp or handle the None case (e.g., treat it as equal) to make rendering robust.

Suggested change

	.partial_cmp(&face_center_depth(b, yaw, pitch))
	.unwrap()
	});
	.total_cmp(&face_center_depth(b, yaw, pitch))
	});