feat(btree): add btree global index reader/writer/factory

[lszskye]

Add BTree Global Index Reader with Lazy Filtering Support

Summary

Implement BTree-based global index reader with lazy file filtering, complementing the existing BTree global index writer. This enables efficient predicate evaluation over BTree global index files with metadata-based file pruning to minimize I/O.

New Classes

BTreeGlobalIndexer — Top-level indexer implementation for BTree global index. Instead of building a B-Tree directly in memory, it forms a logical B-Tree via multi-level metadata over SST files, significantly reducing memory pressure during index reads. Responsible for creating corresponding Writer and Reader instances.

BTreeGlobalIndexReader — Single-file reader for BTree global index. Evaluates filter predicates (Equal, Range, In, StartsWith, etc.) against an SST file structure where each key maps to a list of row IDs. Supports null bitmap handling and min/max key pruning for fast short-circuiting.

BTreeGlobalIndexWriter — Writer for BTree global index files. Writes keys in monotonically increasing order into SST files, merging entries with identical keys into compact row ID lists. Null keys are tracked via a separate RoaringBitmap, serialized and appended to the file end on close.

BTreeGlobalIndexerFactory — Factory class for BTree global indexers. Implements the GlobalIndexerFactory interface and creates BTreeGlobalIndexer instances from configuration options.

LazyFilteredBTreeReader — Lazy-loading, file-filtering BTree global index reader for multi-file scenarios. Uses BTreeFileMetaSelector to prune candidate files based on predicate conditions before lazily instantiating per-file readers on demand. Supports reader caching and parallel execution.

New Tests

• btree_compatibility_test.cpp
• lazy_filtered_btree_reader_test.cpp