Add setTimeout fallback to yieldToMain for hidden/backgrounded tabs

[Copilot]

The yieldToMain() utility relied solely on requestAnimationFrame, which browsers heavily throttle when tabs are hidden. This could stall streaming parse operations if users background the tab during analysis.

Changes

• Added document visibility detection in yieldToMain()
• Falls back to setTimeout(resolve, 0) when document is hidden or rAF unavailable
• Preserves rAF behavior for visible pages (more efficient for UI updates)

export function yieldToMain(): Promise<void> {
  return new Promise(resolve => {
    const hasRaf = typeof requestAnimationFrame === 'function';
    const isDocumentHidden = 
      typeof document !== 'undefined' && (document as any).hidden === true;

    if (!hasRaf || isDocumentHidden) {
      setTimeout(resolve, 0);
      return;
    }
    requestAnimationFrame(() => resolve());
  });
}

This ensures streaming operations progress regardless of tab visibility while maintaining optimal rendering behavior for active tabs.

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