Add loadingCount parameter to GSplat frame:ready event

[mvaligursky]

This PR enhances the frame:ready event in the GSplat component system by replacing the boolean loading parameter with a numeric loadingCount parameter that reports the exact number of assets being loaded.

• Provides granular information about loading state (exact count vs boolean)
• Enables progress tracking for loading screens and UI feedback
• Non-breaking: existing code using if (!loading) works with if (!loadingCount)

Progress Tracking Example

// Track loading progress (0..1)
let maxLoadingCount = 0;
app.systems.gsplat.on('frame:ready', (camera, layer, ready, loadingCount) => {
    maxLoadingCount = Math.max(maxLoadingCount, loadingCount);
    const progress = maxLoadingCount > 0 ? (maxLoadingCount - loadingCount) / maxLoadingCount : 1;
    console.log(`Loading progress: ${(progress * 100).toFixed(1)}%`);
});