Adds an optional foveated mode to the GPU-sort (hybrid) gsplat renderer's contribution culling: the minContribution threshold is raised radially from the screen centre, so low-impact splats are culled increasingly toward the screen edges — where peripheral overdraw dominates XR frame cost — while the centre is untouched.

The effective threshold is:

minContribution + foveationStrength * smoothstep(foveationCenter, 1.0, length(ndc))

with r = length(ndc) being 0 at the screen centre, 1 at the edge (corners beyond 1 get full strength).

Changes:

• GSplatParams.foveationStrength (default 0 = off) and GSplatParams.foveationCenter (default 0.3, the protected-centre radius in NDC units). Only used by the GSPLAT_RENDERER_RASTER_GPU_SORT renderer.
• The values flow through GSplatManager → projector dispatch params → projector UBO → the shared computeSplatCov cull, which applies the radial boost. With strength 0 the cull is bit-identical to before (additive 0); no shader variant is added — the cost is a few ALU once per splat in the projection compute pass.
• Stereo XR: culling is evaluated from eye-0 NDC (consistent with the existing shared stereo cull); the projector's pick variant shares the same uniforms so picking stays consistent with the rendered set.
• The compute-local renderer is unaffected — it passes inert constants (strength 0) to the shared projection helper.

API Changes:

• GSplatParams#foveationStrength — new property (number, default 0).
• GSplatParams#foveationCenter — new property (number, default 0.3).

Examples:

• gaussian-splatting-xr/vr-lod: new in-XR HUD number row to tune the strength live (+/- 5, clamped 0–50).

Performance:

• Intended as a cheap complement to fixed (resolution) foveation on standalone XR devices: fewer surviving splats at the periphery → fewer quads sorted/rasterized → less peripheral overdraw, which profiling on Quest 3 showed to be the dominant frame cost.