[Apple 4/5] Port anatomy rendering tools with RealityKit and targeted Metal shaders

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Goal

Reach feature parity for the anatomy-specific rendering tools while keeping RealityKit as the primary renderer and using Metal only for effects that require it.

Scope

• Implement clipping/cross-section visualization.
• Implement isolation/fade, selection glow, wireframe or structural overlay, depth-correct hotspots, and compare presentation.
• Use RealityKit materials for standard paths.
• Use RealityKit CustomMaterial Metal surface or geometry functions for effects that cannot be expressed efficiently with stock materials.
• Precompile or preload custom material programs to prevent interaction hitches.
• Preserve depth correctness and legibility in stereo on Vision Pro.

Acceptance criteria

• All anatomy tools behave correctly across supported platforms.
• Cross-sections and callouts remain depth-correct in the immersive visionOS scene.
• Custom Metal code is narrowly scoped, documented, and covered by graceful capability fallbacks where necessary.
• Switching tools does not create repeated entities, materials, or shader programs per frame.
• Rendering remains visually stable under motion and close inspection.

Sequence

This is step 4 of 5 and depends on #19 and #20.