fix: measure shrink children via dry layout (#85)

[andyhorn]

Summary

• Replace getMinIntrinsicHeight/Width(infinity) with child.getDryLayout(...) for ResizableSize.shrink() measurement
• Use tight cross-axis + loose-bounded main-axis constraints (cap = parent max − pixels − dividers) so descendant Expanded widgets work and SingleChildScrollView reports its actual content size
• Cache shrink sizes from the measurement pass so _getChildConstraints does not re-measure

Fixes #85.

Background

SingleChildScrollView reports 0 for intrinsic main-axis dimension on its scroll axis. The previous code called getMinIntrinsicHeight(double.infinity) for shrink sizing, so any shrink child wrapping a scroll view collapsed to zero. Intrinsics also passed double.infinity on the cross-axis, which is incorrect — the cross-axis is tight from the parent.

Test plan

• flutter test — 80/80 pass (79 existing + 1 new)
• New regression test: vertical resizable with ResizableSize.shrink() wrapping SingleChildScrollView(child: SizedBox(height: 150)) reports 150

Known limitation

Nested ResizableContainer as a shrink child still returns 0, since ResizableLayoutRenderObject does not override computeDryLayout. Worth a follow-up.

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