Stream and render live Gaussian splats

[suhanpark]

Summary

Stream bounded Gaussian snapshots during optimization and render final/live PLY results with a native screen-space Gaussian rasterizer.

Why

The previous viewer approximated each Gaussian with a low-poly sphere, capped large results, and could only show a result after optimization completed. That produced visibly incorrect geometry and no useful live optimization view.

Changes

• Publish atomic training status and periodic PLY previews
• Cap live previews at a deterministic 50,000-splat scene-wide sample while keeping final exports complete
• Commit Modal Volume preview updates before advertising a new version
• Replace polygonal ellipsoids with Babylon's depth-sorted anisotropic Gaussian renderer
• Preserve full PLY spherical-harmonic color data
• Avoid retaining duplicate large PLY buffers in browser memory
• Add focused tests for the live preview bridge

Validation

• python -m pytest tests/splat/test_live.py tests/visualization/test_runtime.py -q (40 passed)
• pnpm build
• node --check visualization/static/viewer.js
• Converted the existing 1,022,014-splat Lund Door PLY with all three SH textures
• git diff --check

Stack

Depends on #1123. This draft targets studio/react-workspace so the review contains only live-preview and renderer changes. Retarget it as the lower PRs merge.