feat: native 3D splat scene rendering

[Sportinger]

Summary

• Adds the native shared 3D scene path for splats, planes, meshes, models, text, and scene cameras.
• Improves Gaussian splat rendering quality, GPU sorting, per-clip settings, and multi-splat depth handling.
• Adds FPS camera speed stepping and wheel-speed control while moving or looking.
• Bumps release metadata to 1.5.9 and documents the shipped changes.

Verification

• npm run build
• npm run lint (0 errors, existing warnings remain)
• npm run test (102 files, 2098 tests passed)

Remote security checks intentionally not waited for per request.