Support committing multiple Bitmap segments

[Xuanwo]

Summary

Make Bitmap work end-to-end when multiple physical Bitmap segments are published through commit_existing_index_segments(...).

Part of #6862.

Requirements

• Committed Bitmap segments with the same logical index name should be queryable as one logical scalar index.
• Bitmap multi-segment query results should match full scan results.
• describe_indices() should expose the committed Bitmap segments.
• Fix any Bitmap-specific gaps such as BitmapIndex::calculate_included_frags being unimplemented.

Acceptance criteria

• Tests cover equality, range, IN, and null-sensitive Bitmap queries over multiple segments.
• explain_plan() shows scalar index usage where expected.
• describe_indices() reports multiple segments for the new path.
• Existing merged Bitmap indices remain readable and queryable.