[YPARK]

Bug fixes to merge-backend, split out of the MCP work so they can ship now.

• Zipped output. -o m (zip is on by default) left the working m.zarr and never made the m.zarr.zip the help promises: finalize took the bare output instead of the resolved one.
• Output that is an input. -o pbmc beside pbmc.zarr.zip, or -o a --no-zip beside a.zarr, is now refused before anything is cleared. The output is deleted before the inputs are read and zipped over afterwards, so the first fix would otherwise overwrite the input.
• Mismatched rows. Backends whose row names differ hit an assert_eq! panic that printed every name. They now get an error naming the two files, raised before anything is written. The first file is the reference even when it has no rows.
• Batch labels. These go to {output}.batch.gz with the backend suffix stripped; -o m.zarr.zip used to write m.zarr.zip.batch.gz. This applies the same way in merge-backend, merge-mtx and squeeze's merge, which now share the end-of-merge step in one place. A column with no known batch takes the output's whole name (sample.v2).
• merge-mtx backend. merge-mtx writes the backend the output's name resolves to (an .h5 name, or the zarr fallback when hdf5 isn't built), not the one the flag names.

Behaviour changes to note:

• Batch file name: the new name above. Pipelines that pass the old {output}.zarr.zip.batch.gz name explicitly will need to update.
• Explicit .zarr output: -o m.zarr without --no-zip now ends as m.zarr.zip, like every other command.

Tests: new regression tests for the archive, the batch labels, the refusals and mismatched rows. cargo fmt, cargo clippy --all-targets and cargo test are clean.

Left for follow-ups, all pre-existing:

• union-mode squeeze rejects a bare -o prefix, and only after loading every input;
• find_aligned_rows counts duplicate rows within one input as common;
• merge-backend with a single input exits 0 and writes nothing;
• from_h5ad panics (assert_eq!) on short name arrays;
• move the merges onto zarr_io::prepare_output/finalize_output.