Fix Style::operator== ignoring boxSizing

[kwy404]

Summary

yoga::Style::operator== compares every style field except boxSizing_ (the field was added in #1701 but never added to the comparison). Two styles that only differ in box sizing therefore compare equal.

Effects:

• YGNodeCopyStyle(dst, src) only copies when dst->style() != src->style(). If src only differs in box sizing, nothing is copied and dst keeps its old box sizing without being marked dirty.
• Code that compares styles to decide whether a node needs layout misses the change. For example React Native's YogaLayoutableShadowNode::updateYogaProps only dirties the node when styleResult != yogaNode_.style(), so a prop update that only changes boxSizing is not detected there.

The fix compares boxSizing_ together with the other enum fields.

Changelog: [General][Fixed] - Fix YGNodeCopyStyle and style equality ignoring boxSizing

Test Plan

Added copy_style_box_sizing to tests/YGStyleTest.cpp. It copies a style that only sets YGBoxSizingContentBox onto a default node.

Before the fix:

[ RUN      ] YogaTest.copy_style_box_sizing
tests/YGStyleTest.cpp(60): error: Expected equality of these values:
  YGBoxSizingContentBox
    Which is: 1
  YGNodeStyleGetBoxSizing(node0)
    Which is: 0
[  FAILED  ] YogaTest.copy_style_box_sizing (0 ms)

After the fix it passes, and the full C++ suite (MSVC 2022 + Ninja) passes: 855 tests. clang-format 21.1.2 (the version pinned in scripts/clang-format) reports no changes.