Convert: h5ad ↔ zarr

[Claptar]

Part of #20. Depends on #21.

Goal

adata convert data.h5ad -o data.zarr [--zarr-format 2|3]
adata convert data.zarr -o data.h5ad

This already works implicitly through adata subset in.h5ad -o out.zarr, which proves copy_store_contents handles every backend pair. A dedicated, discoverable command is what's missing.

Plan

• Thin converter over storage.copy_store_contents.
• Keep the source's chunking and compression by default; add --rechunk and --compressor for zarr targets.
• Write consolidated metadata for zarr targets.
• zarr v3 by default from an h5ad source (existing behaviour); follow the source format for zarr → zarr.
• Refuse -o - and output == input.

Tests

• All six pairings of h5ad / zarr2 / zarr3.
• Open the result with anndata.read_zarr / read_h5ad under the integration marker.
• Perf guard: one read per chunk, no full materialisation.