Add initial SOG format support

[querielo]

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Issue: https://github.com/mkkellogg/GaussianSplats3D/issues/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.