Interface SplatTileGpu

A splat tile decoded straight into GPU data-texture form. The three texture arrays are byte-identical to packPositions / packAttribs / packSh from @bitruvius/splats packing.ts (the shader parity oracle), so a renderer could upload them directly:

• posTex RGBA32I count texels: [x, y, z, 0]
• attrTex RGBA32UI count texels: scale/rotIdx | rotAB | rotC/opacity/colorR | colorGB
• shTex RGBA8UI count·ceil([0,9,24,45][deg]/4) texels (empty at deg 0)