Function generateSyntheticGSplats

Generate a clustered gsplats scene as an InstancedGSplatsMeshConfig.

Each splat gets a lower-triangular Cholesky factor L with strictly positive diagonal — by construction a valid factor of the SPD covariance Σ = L·Lᵀ:

• characteristic size log-uniform over one decade ([0.2 %, 2 %] of bounds) → scale varies;
• per-axis diagonal jitter ×[0.4, 1.6] → anisotropy varies;
• off-diagonals ±0.8 × the row's diagonal → orientation/correlation varies (Σ gains substantial off-diagonal terms). Scale AND orientation therefore differ across splats, so depth-sorted 'normal' blending is measurably order-dependent.

Amplitudes are positive and varied ([0.2, 1.0)); colors vivid random RGB.

Packing matches SplatTexelSource.choleskyFactors: 6-stride row-major [L00, L10, L11, L20, L21, L22] per splat.