Add new 3DGS walkthrough demo

[nsalminen]

Adds a new 3DGS scene walkthrough demo that lets users explore room-scale environments captured with 3D Gaussian Splatting. The demo ships with two scenes derived from the Zip-NeRF dataset and optimized to be lightweight enough to render smoothly across a range of XR devices. More scenes can be added in the future without significant code changes.

This PR also exports SparkRendererHolder from the SDK, which is needed by any app that creates its own SparkRenderer and wants the simulator's color space correction (introduced in #37) to apply correctly.

Features

• Two room-scale 3DGS scenes (NYC, Alameda) served from the proprietary assets CDN, based on Zip-NeRF captures
• Smooth crossfade transitions between scenes via a 1.5s long pinch or number keys (1, 2)
• WASD locomotion in XR via reference space offset
• Correct color space handling — creates and registers a SparkRenderer so the simulator can toggle encodeLinear as introduced in Fix color space issue for gaussian splats. #37
• SparkRendererHolder exported from xrblocks for use in external demos that manage their own SparkRenderer
• Docs page added to the Advanced Demos section