perf(Breadcrumbs): Optimize CSS :has() selector and batch offsetWidth reads

[mattcosta7]

Summary

Performance optimizations for Breadcrumbs component to improve INP and reduce layout thrashing.

Changes

1. CSS :has() selector optimization - Scope :has(.MenuOverlay) to shallow path > .MenuDetails .MenuOverlay for O(1) lookup
2. Batch offsetWidth reads - Read all widths in a single pass to avoid forced synchronous layouts

Expected INP Impact

Scenario	Before	After	Improvement
Worst case (many breadcrumb items, overflow menu)	~40-60ms (style + layout)	<10ms	75-85% reduction
Average case (typical breadcrumbs, 3-5 items)	~15-25ms	<5ms	65-80% reduction
Best case (few items, no overflow)	~5-10ms	<2ms	60-80% reduction

Why this matters

• :has() optimization: MenuOverlay detection was scanning entire subtree on every interaction
• Layout batching: Reading offsetWidth twice in sequence forces two layout calculations. Batching into one pass eliminates layout thrashing during resize calculations.

---

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

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🦋 Changeset detected

Latest commit: ee72d57

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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