Add segmented BTree index planning and merge support

[zhangyue19921010]

Summary

This PR adds segmented index plan, merge, and optimize support for BTree scalar indexes.

What Changed

• Adds a BTree implementation for the segmented index builder.

  • Supports planning staged BTree segments.
  • Supports 1:1 passthrough when segments do not need consolidation.
  • Supports N:1 consolidation when target_segment_bytes is set.

• Adds BTree support to merge_existing_index_segments.

  • Merges multiple existing BTree segments into one physical segment.

• Adds a lower-level BTreeIndex::merge_segments API.

  • Merges multiple BTree indexes plus optional new training data.

• Extends scalar index optimize support for multi-segment BTree indexes.

  • Can merge multiple existing BTree segments with newly appended fragments.