feat: delegate feature persistence to copick

[uermel]

Tracks #86 and is the fourth PR in the copick-utils Zarr v3 migration stack.

Base: uermel/zarr-v3-u2-delegated-writers

This replaces the legacy root-array/Blosc feature writer while preserving its public API and its frozen result for inputs that already worked:

• keeps every existing parameter, order, default, and positional call valid;
• adds optional keyword-only chunks=None and shards=None controls;
• preserves chunk subdivision, scalar overlap, transpose, and edge placement;
• corrects the pre-existing trailing-overlap slice bug by deriving bounded read, crop, and output slices independently on every axis;
• stages the complete feature-major float32 tensor in a temporary np.memmap instead of process memory;
• releases and removes staging after success or failure;
• performs exactly one final CopickFeatures.from_numpy() write; and
• delegates metadata, chunks, shards, codecs, overwrite, and validation to copick core.

Targeted tests preserve the frozen pre-migration digest, cover the formerly crashing boundary geometry on all three axes, detect unwritten cells, verify staging cleanup and one-write behavior, and cap peak Python memory below both 64 MiB and 75% of the logical 30-channel output size. Broader converter and layout coverage remains deferred.

Validation: all 63 behavior tests and pinned pre-commit hooks pass. The full suite passes on Python 3.11.15/Zarr 3.1.6 and Python 3.12.10/Zarr 3.3.0.

The next stack layer is uermel/zarr-v3-u4-release-gates.