Single-part group fills now delegate to Fill(NestItem) which runs
the full strategy pipeline, eliminating ~70 lines of duplicated
manual phase logic. Multi-part group fills retain the linear
pattern fill (unique to multi-part groups).
PairFiller now references FillHelpers directly instead of
bouncing through DefaultNestEngine helper methods.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>