Replace the monolithic RigNonUniformRefinement class with a proper
polymorphic hierarchy: RigRefinement (abstract base), RigNoRefinement
(identity), RigUniformRefinement (O(1) uniform), and
RigNonUniformRefinement (per-cell custom fractions).
Ownership uses std::unique_ptr<RigRefinement>, consumers receive
const RigRefinement&. This eliminates the misleading class name,
removes effectiveRefinement() indirection, and gives each refinement
mode an appropriately optimized implementation.
Add NonUniformSubMode enum (Custom Widths, Linear Equal Split, Logarithmic
Towards Center) to give users simpler alternatives to manually entering
comma-separated fractional widths.
Linear mode splits each cell in the range into N equal subcells.
Logarithmic mode auto-computes widths with finer resolution near the
center of the range using a geometric series.