Wraps PairFiller in an IFillStrategy so the pairs phase participates
in the pluggable pipeline. Stores BestFitResults in SharedState for
downstream strategies (Extents) to reuse.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Move BuildRotatedPattern and FillPattern static methods into a new
public FillHelpers class in Strategies/. DefaultNestEngine retains
internal static forwarding stubs so existing callsites are unchanged.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
