Add initial SOG format support

[querielo]

Issue: #476

This PR adds first-class support for the SOG (Spatially Ordered Gaussians) scene format to GaussianSplats3D. It enables loading both:

• A bundled .sog archive (ZIP with stored entries), and
• A multi-file SOG directory containing meta.json and referenced WebP images.

Limitations and follow-ups

• Zip reader supports only STORE (no DEFLATE). This matches the splat-transform writer.
• SH degree 3 data is currently clamped to degree 2. Follow-up: import all 15 AC coefficients per channel when viewer-side support is in place.
• Consider workerizing SOG parsing for large scenes to keep the main thread responsive.

[Copilot]

Pull Request Overview

This PR adds initial support for the SOG (Spatially Ordered Gaussians) scene format to GaussianSplats3D, enabling loading of both bundled .sog archives and multi-file SOG directories.

Key changes include:

• Implementation of a minimal ZIP reader for browser-based .sog archive extraction (STORE method only)
• SOG parser that reconstructs 3D Gaussian splats from WebP-encoded data using quantization and codebooks
• Integration with existing loader infrastructure and viewer components

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/loaders/sog/ZipReaderBrowser.js	Minimal ZIP reader supporting STORE compression for .sog archives
src/loaders/sog/SogParser.js	Core parser for SOG format data reconstruction from WebP images
src/loaders/sog/SogLoader.js	Main loader with support for both directory and ZIP-based SOG files
src/loaders/Utils.js	Added SOG format detection for .sog file extensions
src/loaders/UncompressedSplatArray.js	Fixed spherical harmonics data placement in splat components
src/loaders/SceneFormat.js	Added SOG format enum value
src/index.js	Exported SogLoader for public API
src/Viewer.js	Integrated SOG loading support into main viewer

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[ShunChengWu]

The code here createed a sequential layout [R0, R1, R2, ..., G0, G1, G2, ..., B0, B1, B2, ...], but the renderer expects an interleaved lauout.
The code here should be changed to something like this:

const shIndexMap = [
    0, 1, 2, 9, 10, 11, 12, 13, 24, 25, 26, 27, 28, 29, 30,  // R channel indices
    3, 4, 5, 14, 15, 16, 17, 18, 31, 32, 33, 34, 35, 36, 37,  // G channel indices
    6, 7, 8, 19, 20, 21, 22, 23, 38, 39, 40, 41, 42, 43, 44   // B channel indices
];
for (let j = 0; j < 3; j++) {
  for (let k = 0; k < shNCoeffsWanted; k++) {
    const u = (label % 64) * shNCoeffsTotal + k;
    const v = Math.floor(label / 64);
    if (u < centroidsImg.width && v < centroidsImg.height) {
      const cidx = (v * centroidsImg.width + u) * 4;
      const codebookIdx = centroidsImg.data[cidx + j];
      const outIndex = shIndexMap[j * 15 + k];
      if (outIndex < restCount) {
        rest[outIndex] = shNCodebook[codebookIdx] ?? 0;
      }
    }
  }
}