fix: preserve untouched fragments in slow merges

[FANNG1]

Summary

• preserve untouched fragment metadata when merge_columns_df uses the slow path with partial-fragment input
• share the fragment-completion logic with the existing fast path
• add a four-fragment regression test covering partially touched and completely untouched fragments

Problem

LanceOperation.Merge builds the new manifest from the fragment metadata passed to it. The slow path previously passed only metadata returned by fragment.merge(...), so fragments absent from the input DataFrame disappeared from the committed dataset version.

The fix completes the metadata list from the driver dataset snapshot before committing. The existing read_version check continues to reject concurrent changes rather than mixing snapshots.

Testing

• .venv/bin/pytest -q tests/io/lancedb/test_lance_merge_evolution.py tests/io/lancedb/test_fast_path_merge.py (44 passed)
• Ruff check and format check
• Mypy on daft_lance/lance_merge_column.py
• git diff --check

Fixes #55

[universalmind303]

thanks again @FANNG1