Support datatree roots with consolidated metadata

[ljstrnadiii]

Description

Some Zarr sources are datatree-style roots where the target variable lives under child groups (for example region_a/<variable>, region_b/<variable>) instead of directly at root. Today, zarr-layer assumes a single dataset path and does not handle this structure well.

Goals

• Auto-detect datatree child datasets from root consolidated metadata.
• Initialize one store/mode per child dataset and render them as one logical layer.
• Keep single-dataset behavior unchanged.
• Support viewport-based child culling when bounds are known.
• Support query routing for points and merged query results for polygons across matching children.
• Improve CRS handling for child datasets when CRS/proj4 metadata is incomplete.

Notes / Constraints

• Assume child footprints do not overlap.
• Assume scale_factor / add_offset are consistent across children.
• Prefer fast bounds discovery from child multiscales layout in root consolidated metadata.
• If child layout metadata is missing, fall back to per-child coordinate-array bounds detection (slower).

Acceptance Criteria

• Datatree root sources render correctly without custom per-child wiring.
• Existing non-datatree sources behave exactly as before.
• Documentation includes datatree behavior and performance implications (fast metadata path vs coordinate fallback).