Fix unbounded stretch crash, accessibility order, and doc typo (#45, #46, #48)

[RandalSchwartz]

Important

This request is being generated by antigravity using gemini-3-flash, and should be viewed with caution.

This pull request addresses several outstanding issues in the boxy package:

Fixes

1. Unbounded Cross-Axis Stretch Crash (#48)

Fixed a layout crash in RenderBoxyFlex where CrossAxisAlignment.stretch would attempt to apply infinite constraints when the cross-axis was unconstrained. This now mirrors the behavior of Flutter's RenderFlex.

2. Accessibility / Semantics Traversal Order (#45)

• Added visitChildrenForSemantics to BaseBoxyDelegate, allowing delegates to control the order in which children are visited by screen readers.
• Updated RenderBoxyMixin to delegate semantics visitation to the delegate (defaulting to visitation of all non-ignored children).
• Implemented visual-order semantics traversal for RenderBoxyFlex.
• Added a new semantics phase to BoxyDelegatePhase to allow safe access to render objects during semantics updates.

3. Documentation Typo in BoxyId Example (#46)

Corrected a copy-paste error in the BoxyId documentation where worldSize.height was used instead of worldSize.width in a Size calculation.

Verification

• Ran existing tests and verified that a reproduction case for the semantics issue now passes.
• Verified the fix for the unbounded stretch crash using a local reproduction test case.

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