Export the RFC-5 metadata dataclasses at the package top level

[vboussot]

The RFC-5 metadata dataclasses are only importable from an internal path. Axis is exported at the package top level, but CoordinateSystem, CoordinateSystemIdentifier and Displacements must come from ngff_zarr.v06.zarr_metadata (checked at 0.45.0).

Code that builds RFC-5 coordinate systems and displacement declarations (the write side of what #708 covers) therefore imports an internal module that any refactor can move without a deprecation cycle, and it breaks at import time when that happens.

Exporting the three names beside Axis in __init__ would settle it. One-line change; happy to send the PR.

[vboussot]

Triage while implementing: a bare top-level export is blocked by a real ambiguity. Both v06.zarr_metadata and v09.zarr_metadata define their own CoordinateSystem (v09 imports Displacements from v06 but redefines the coordinate-system model), so from ngff_zarr import CoordinateSystem would have to pick a spec version and silently change meaning when the default moves.

So the options are: (a) version-scoped public paths, i.e. declare ngff_zarr.v06.zarr_metadata / v09.zarr_metadata public API and pin their names with a test, or (b) top-level exports under version-explicit names. Leaving this open for that decision rather than landing the wrong one-liner.