fix(merge): track chunk provenance instead of re-deriving group membership from labels

[jayhesselberth]

Summary

• leech data merge -i A=file1.npz -i B=file2.npz re-derived each group's label set from the original labels values stored in each input file, then matched chunks back into group 1/2 by that value. Two files sharing an internal label (e.g. both prepared with label "Thr" but representing different classes of the actual comparison, like attached ligand) collapsed to the same label_int — silently, with no error, and a log line that still looked correct.
• _parse_and_validate_inputs (src/leech/commands/merge_split.py) now builds relabel_by_file: dict[Path, tuple[int, str]], keyed by which -i argument supplied each file, instead of re-deriving membership from stored labels values (meta_to_chunk_labels is gone, along with the per-file np.load peek it required).
• merge_and_split_chunks / merge_and_kfold_split_chunks (src/leech/splitting/splitter.py) gained a relabel_by_file parameter (mutually exclusive with the existing relabel_pairwise) that's applied directly as the label override, bypassing value-matching entirely — including in the legacy in-memory (output_dir=None) path.
• The value-based relabel_pairwise path is unchanged for process_comparison_spec's TSV workflow, where group membership genuinely is defined by label values drawn from an arbitrary set of files — but it now raises ValueError if the caller's group1/group2 label sets overlap, instead of silently collapsing to one class.

Closes #323

Test plan

• uv run pytest tests/test_splitter_merge.py -v — 30 passed, including new regression tests reproducing the exact bug (TestMergeSplitCLIProvenance) and the guard-rail coverage (test_relabel_pairwise_overlapping_groups_raises, test_relabel_pairwise_and_relabel_by_file_are_mutually_exclusive)
• uv run pytest tests/test_comparison_spec.py -v — 7 passed (confirms the untouched value-based TSV path still works)
• uv run pytest tests/test_mask_seq_side.py -v — 24 passed (exercises other merge_split.py code paths)
• uv run ruff format --check . / uv run ruff check . on touched files — clean
• uv run ty check src/leech/ — clean

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