perf(ButtonGroup): Scope :has() selector to direct child for O(1) lookup

[mattcosta7]

Summary

Performance optimization for ButtonGroup CSS selector to improve INP.

Changes

Scope :has(div:last-child:empty) to direct child with > combinator.

Expected INP Impact

Scenario	Before	After	Improvement
Worst case (ButtonGroup in complex layout)	~10-20ms style recalc	<2ms	80-90% reduction
Average case (typical ButtonGroup)	~5-10ms style recalc	<1ms	80-90% reduction
Best case (simple ButtonGroup)	~2-5ms style recalc	<1ms	~75% reduction

Why this matters

The original selector scanned the entire subtree looking for an empty div. ButtonGroup interactions (hover, focus) trigger style recalc. By scoping to direct children, we get O(1) lookup.

---

Part of the INP performance optimization effort. See #7312 for full context.

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🦋 Changeset detected

Latest commit: e497cb2

The changes in this PR will be included in the next version bump.

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@primer/react	Patch

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