Both formats store identical raw little-endian numeric payloads on
disk. This conversion re-serializes only the JSON metadata and the
string blobs (which differ between formats: files-format uses
newline-delimited UTF-8, while Zarr uses vlen-utf8).
Details
Same-filesystem only. The function probes a hard-link from src to
dst up front; if the link fails (typically because the paths are
on different filesystems) the call errors with no automatic copy
fallback.
Note: this implementation is correctness-first; the numeric blobs
are re-written through the regular set_* API rather than
hard-linked. The probe still enforces the same-filesystem
constraint so a future hard-link optimization can ship without an
API break.