Apply after_stat mappings: rename stat-computed columns to canonical
aesthetic names. Called after the stat step in the build pipeline.
DataAttr/Tooltip targets are validated and routed the same way as in
resolve_mappings (built directly and pushed with
MappingStage::AfterStat is obscure but not impossible, so it gets the
same gate).
Evaluate aesthetic mappings: extract columns from source data
and rename them to canonical aesthetic names (x, y, color, etc.).
Only resolves BeforeStat mappings.