Add Bitmap support to IndexSegmentBuilder

[Xuanwo]

Summary

Wire Bitmap into the common IndexSegmentBuilder planning and build flow.

Part of #6862.

Requirements

• IndexSegmentBuilder::plan() should accept IndexType::Bitmap.
• Single-input Bitmap plans should avoid unnecessary rewrites where possible.
• Multi-input Bitmap plans should call the Bitmap segment merge path.
• build_all() should work for Bitmap segments.

Acceptance criteria

• Bitmap can run through create_index_segment_builder().with_index_type(IndexType::Bitmap).with_segments(...).build_all().
• Pass-through plans preserve usable segments.
• Multi-input plans produce commit-ready Bitmap segments.
• Existing vector and inverted segment builder behavior is unchanged.