Add MiniStats stat presets for gsplat stats

[mvaligursky]

Summary

• Add MiniStats.statPresets static property with predefined stat groups (gsplats, gsplatsCopy)
• Extend MiniStats.getDefaultOptions() to accept an optional extraStats array of preset names, inserting them after the 'Frame' stat entry
• User-specified array order matches visual top-to-bottom display order
• Simplify gsplat examples (lod-streaming, flipbook) to use the new preset API instead of manual option splicing

Public API Changes

Before:

const msOptions = pc.MiniStats.getDefaultOptions();
const frameIndex = msOptions.stats.findIndex(s => s.name === 'Frame');
msOptions.stats.splice(frameIndex + 1, 0, {
    name: 'GSplats',
    stats: ['frame.gsplats'],
    decimalPlaces: 3,
    multiplier: 1 / 1000000,
    unitsName: 'M',
    watermark: 10
}, {
    name: 'GsplatsCopy',
    stats: ['frame.gsplatBufferCopy'],
    decimalPlaces: 1,
    multiplier: 1,
    unitsName: '%',
    watermark: 100
});
const miniStats = new pc.MiniStats(app, msOptions);

After:

const miniStats = new pc.MiniStats(app, pc.MiniStats.getDefaultOptions(['gsplats', 'gsplatsCopy']));

Backward compatible — getDefaultOptions() with no arguments returns the same result as before.