Adds a new Gaussian Splatting example: a first-person walkthrough of a splat scene with a Depth of Field post effect.

Changes:

• New gaussian-splatting/depth-of-field example (.example.mjs + .controls.jsx) and its thumbnails.
• The DOF focus depth is supplied by a lightweight proxy mesh rendered into the camera depth prepass only (excluded from the forward/color pass), so it contributes depth without being visible.
• A separate simplified mesh is used as a static triangle-mesh collider for the first-person controller.
• Autofocus: raycasts the collision mesh at the screen center and eases the focus distance there, with an auto-derived focus range and an HTML reticle.
• Fast time to first frame: starts at the coarsest LOD and restores the full range (and enables DOF + sharpening) once the first frame is ready.
• Proxy and collision meshes are Draco-compressed; the splat streams from a remote LOD/SOG URL.

Examples:

• gaussian-splatting/depth-of-field

Depends on MeshInstance.shaderPassMask (#8852, merged).