feat(exporting): add OpenUSD USDA and USDZ geometry export

[JarrettSJohnson]

Summary

Implements the OpenUSD portion of #497 by adding USDA and USDZ export for currently displayed geometry. The exporter preserves scale (metersPerUnit = 1e-10), colors, normals, opacity, and geometry_export_mode for spheres, sticks, sausages, cones/frustums, ellipsoids, cartoons, and surfaces.

USDZ export uses the external OpenUSD usdcat command to create a binary USDC layer and packages it as one uncompressed, 64-byte-aligned scene.usdc member. Export is available through cmd.save, cmd.get_usda, and the GUI. Ramp colors are exported black with a warning; transparent solids may be tessellated for rendering fidelity.

Testing

• Added coverage for primitive geometry, colors, opacity, export modes, USDZ packaging, and error handling.
• 38 focused tests pass (1 incentive-only skip).
• Validated exported packages with usdchecker --arkit and Apple usdrecord.
• All CI checks pass.