[fsspec-aware #479] zarr: make init_from_disk read path fsspec-aware (read, priority 2)

[xroynard]

Sub-task of #479. Read side, zarr backend — easy, mirror of the write-side zarr task. Covers parts ① + ② of the read-side analysis (issuecomment-5166383239).

Goal

Let zarr/reader.py::init_datasetdict_from_disk read from any fsspec URL, not just local disk.

Current state (verified on main)

zarr.open(zarr.storage.LocalStore(local_path / sn), mode="r")

Two hardcoded-local spots in the same function:

1. Store — pinned to LocalStore. zarr.open() accepts an fsspec URL directly in read mode (checked: zarr.open("memory://…", mode="r") round-trips), so this is just selecting the store from the path/storage_options.
   • ⚠️ Trap: LocalStore("s3://…") does not raise — it treats the string as a literal local path and reads the wrong place. This is a correctness fix, not an exception to guard.
2. Split discovery — local_path.iterdir() (is_dir() filtering) is local-only; on a remote target it must go through fs.ls(...) on the fsspec filesystem derived from the URL.

Scope

• Select the store from the URL/storage_options.
• Replace iterdir() split discovery with fsspec fs.ls(...).

Tests

• Round-trip on memory:// (write via the zarr writer, read back via init_from_disk).

Priority

★ (2/5) — small, high value, testable on memory://; symmetric with the write-side zarr task.