Introduces IterativeShrinkFiller.Fill, which composes RemnantFiller and
ShrinkFiller by wrapping the caller's fill function in a closure that tries
both ShrinkAxis.Height and ShrinkAxis.Width and picks the better FillScore.
Adds IterativeShrinkResult (Parts + Leftovers). Covers null/empty inputs and
single-item placement with three passing xUnit tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
