Adds a new Gaussian-splat example that joins two LOD-streamed splat scenes with a stencil-masked, walk-through portal. The Roman Parish scene and Skatepark scene are aligned by per-scene "portal anchors", so crossing the portal plane swaps which scene is "current" without any visible teleport. A translucent glass overlay sits inside the portal opening, and procedural snow gsplats are bound to the Skatepark layer (so they appear naturally in that world and through the portal).

Examples:

• New examples/src/examples/gaussian-splatting/splat-portal.example.mjs with companion splat-portal.controls.mjs (shows current world + gsplat count).
• Thumbnails added for the new example (gaussian-splatting_splat-portal_{large,small}.webp).

Technical notes:

• Stencil-masked portal: invisible mask plane writes stencil + depth at the portal plane; the visible arch clears stencil over its silhouette so the through-scene only renders inside the actual opening.
• Per-frame material flip on the two splat layers: the "through-scene" layer gets portalOnly stencil + FUNC_GREATER depth (oblique near-clip at the portal plane); the "from-scene" layer gets no stencil + FUNC_LESSEQUAL, letting foreground splats correctly occlude both the portal and the glass overlay.
• Layer composition uses insertOpaque / insertTransparent directly to get an explicit sub-layer render order: mask opaque → arch opaque → through-scene transparent → glass transparent → from-scene transparent.
• Walk-through detection uses a segment-vs-quad intersection in mask-plane local space, so walking around the plane doesn't trigger a swap.
• Snow uses the existing GsplatWeather script bound to the Skatepark layer; a single SNOW_EXTENT controls the volume.