Tracking: Align Bitmap with segmented index lifecycle

[Xuanwo]

Summary

Bitmap should support the same segment lifecycle used by vector-style distributed indexing:

1. create_index_uncommitted(fragment_ids=...) builds a durable, uncommitted physical segment.
2. merge_existing_index_segments(segments) optionally consolidates Bitmap segments.
3. commit_existing_index_segments(index_name, column, segments) publishes the final physical segments as one logical Bitmap index.

This work must be additive. Existing merged Bitmap indices and the legacy distributed Bitmap workflow based on merge_index_metadata(index_uuid, "BITMAP") must continue to work.

Compatibility boundary

Do not change the legacy meaning of index_uuid in the existing Bitmap distributed path. Today it is a shared merge directory / job id for part_*_bitmap_page_lookup.lance files, not a physical segment id. The new canonical segment path must use independent physical segment UUIDs.

Tasks

• Add canonical Bitmap segment build path #6863 Add canonical Bitmap segment build path
• Support committing multiple Bitmap segments #6864 Support committing multiple Bitmap segments
• Add Bitmap support to merge_existing_index_segments #6865 Add Bitmap support to merge_existing_index_segments
• Add Bitmap support to IndexSegmentBuilder #6866 Add Bitmap support to IndexSegmentBuilder
• Preserve and document legacy Bitmap distributed merge #6867 Preserve and document legacy Bitmap distributed merge
• Discuss deprecation and default migration for distributed Bitmap #6868 Discuss deprecation and default migration for distributed Bitmap

Suggested order

1. Build canonical Bitmap segments.
2. Commit and query multiple Bitmap segments.
3. Merge Bitmap segments.
4. Wire Bitmap into IndexSegmentBuilder.
5. Preserve and document the legacy path.
6. Discuss deprecation and default migration separately.

Non-goals for this tracking issue

• Do not remove merge_index_metadata(index_uuid, "BITMAP").
• Do not remove BitmapParameters.shard_id.
• Do not change ordinary create_scalar_index("col", "BITMAP") into a multi-segment build by default.
• Do not change Python defaults before a separate deprecation / migration discussion.