Function loadGSplatsChunkIndex

Probe chunk_bounds under zarrLocation and assemble a ChunkSpatialIndex. Returns null for the soft-fail cases the gsplats facade handles inline:

• attrs.ordering === 'none' — no spatial ordering, full load fallback;
• the chunk_bounds array is missing on disk — full load fallback (datasets without spatial indexing).

Logs a warning (does not raise) on chunk-count mismatches between Math.ceil(n_splats / chunk_size) and the array's element-count implied count, taking the smaller of the two as the truth.