Function loadPointsChunkIndex

Probe chunk_bounds under zarrLocation and assemble a PointsChunkIndex. Returns null for the two soft-fail cases the loader already handled inline:

• the node has no spatial ordering (ordering is undefined or 'none') — the dataset is 3D-without-Morton/Hilbert and the loader should fall back to "load all points";
• the chunk_bounds array is missing or the shape's last dim isn't 2 — same fallback path.

Logs warnings (does not raise) on length mismatches and on dimension-coverage mismatches between ordering_dims ∪ slice_dims and the array's ndim.