feat(python): add Bitmap binding for RoaringBitmap

[wjones127]

Two sites in python/src/transaction.rs copy a RoaringBitmap through a Python set/list on every Rust↔Python crossing:

• IndexMetadata.fragment_bitmap ↔ Index.fragment_ids
• DataOverlayFile's OverlayCoverage ↔ offsets (added in feat(python): expose DataOverlay commit operation #7540), whose bitmap_from_sorted_offsets helper has a comment pointing straight at this issue

This adds a Bitmap type (lance.bitmap.bitmap) wrapping Arc<RoaringBitmap>:

• Constructible from a Python iterable of ints, a range, or an integer pyarrow Array/ChunkedArray (read via a zero-copy buffer, not per-value Python objects).
• __len__/__contains__/__iter__/__eq__, add/discard/update (copy-on-write via Arc::make_mut), and pickling.

Both transaction.rs sites now emit Bitmap on output and accept it directly on input (an Arc clone instead of a Python-set round trip), while still accepting a plain set/list on input for backward compatibility. The same set-expansion pattern existed in two more places — IndexSegment.fragment_ids and IndexSegmentDescription.fragment_ids in indices.rs (used by list_indices()/describe_indices()) — so those were switched to Bitmap too.

FragmentMetadata.to_json() normalizes Bitmap/List[Bitmap] offsets back to plain (nested) lists of ints for JSON serialization.

Fixes #7695

Test plan

• New python/tests/test_bitmap.py: construction (list/range/pyarrow array of various int widths/chunked array), len/iter/in, equality, pickle round-trip, copy-on-write mutation isolation, error cases (negative values, out-of-range values, nulls).
• Extended test_dataset.py overlay tests to cover Bitmap-typed offsets (dense + sparse) and assert the output type; updated the invalid-offsets error-message assertion.
• Extended test_indices.py/test_scalar_index.py to assert Index.fragment_ids/IndexInformation.fragment_ids are Bitmap.
• cargo fmt --all, cargo clippy -p pylance --lib --tests -- -D warnings, uv run ruff format/ruff check, uv run pyright all clean.

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