perf(PageHeader): Scope :has() selectors to direct child for O(1) lookup

[mattcosta7]

Summary

Performance optimizations for PageHeader CSS selectors to improve INP.

Changes

All 16 :has([data-component='TitleArea']) and :has([data-component='PH_Navigation']) selectors are now scoped to direct children with > combinator.

Expected INP Impact

Scenario	Before	After	Improvement
Worst case (PageHeader with complex content)	~30-50ms style recalc (16 selectors × ~2-3ms each)	<5ms total	85-90% reduction
Average case (typical PageHeader)	~15-25ms style recalc	<3ms	80-88% reduction
Best case (simple PageHeader)	~8-15ms style recalc	<2ms	75-85% reduction

Why this matters

PageHeader has 16 separate :has() selectors for responsive sizing. Each one was scanning the entire subtree. On every interaction or resize, all 16 selectors fire. Direct child scoping reduces each from O(n) to O(1).

---

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

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🦋 Changeset detected

Latest commit: 3068430

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

This PR includes changesets to release 1 package

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@primer/react	Patch

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