Add canonical Bitmap segment build path

[Xuanwo]

Summary

Add a canonical Bitmap build path where each create_index_uncommitted(fragment_ids=...) call writes one independent physical Bitmap segment using the standard bitmap_page_lookup.lance layout.

Part of #6862.

Requirements

• Each uncommitted Bitmap build should use its own physical segment UUID.
• The canonical path should not require shard_id.
• The canonical path should not write part_*_bitmap_page_lookup.lance files.
• The legacy shard_id / part_* path must remain unchanged.

Acceptance criteria

• Multiple fragment groups can be built as independent Bitmap segment metadata.
• Each segment reports the correct fragment_bitmap.
• Ordinary single Bitmap index creation remains single-segment.
• Existing distributed Bitmap tests continue to pass.