SelectPanel synchronizes selection and loading state from effects #8607

Description

SelectPanel synchronizes intermediate selection, loading state, and selected-item ordering from passive effects. These effects can call multiple state setters during commit, producing cascading follow-up renders on pages that mount closed or data-backed SelectPanels.

Expected behavior: SelectPanel should preserve controlled/uncontrolled selection and loading semantics without avoidable commit-spawned updates.

Steps to reproduce

1. Render closed SelectPanels with empty and preloaded item arrays in a React 19 profiling build.
2. Change items, open, and controlled selection values.
3. Record commits in Chrome DevTools.
4. Observe state publication from the selection/loading/sort synchronization effects.

Version

@primer/react 38.42.0

Browser

Chrome

Acceptance criteria

• Preserve loading announcements and selected-item ordering.
• Preserve modal intermediate selection behavior.
• Add tests for mount, open/close, item arrival, and controlled selection changes.
• Fix this in Primer React rather than requiring a downstream patch.