perf(yoga): experiment skipping Yoga dirtying for unchanged setSize/setPositionType (RN #59101)

[kosmydel]

Description

Ports react/react-native#59101 behind a new discordSkipUnchangedYogaDirty feature flag (default false). With the flag on, YogaLayoutableShadowNode::setSize and setPositionType return early when the value is unchanged, instead of dirtying the Yoga node on every clone (component descriptors call these from adopt), which otherwise forces a layout pass over the whole tree on each commit.

The flag follows the same shape as discordUiManagerOptimization, so the app can drive it from per-platform experiments.

Test steps

1. The two ported YogaDirtyFlagTest cases in ViewTest.cpp (settingUnchangedSizeMustNotDirtyYogaNode, settingUnchangedPositionTypeMustNotDirtyYogaNode) override the flag to true and assert that re-setting the same size or position type doesn't dirty the tree.
2. With the flag at its default (false), behavior is unchanged.

Note

These gtests are BUCK-only upstream and aren't run by this repo's GitHub workflows, so they weren't executed here. Validation happens in the Discord app via the experiment.