1. GlobalSymbolIndex constructor: Add project_id parameter lookup
- Get project_id from registry using source_root
- Pass project_id to GlobalSymbolIndex constructor
2. Binary cascade search path handling:
- Add VectorMetadataStore import for centralized search
- Fix _build_results_from_candidates to handle centralized mode
- Use VectorMetadataStore for metadata, source_index_db for embeddings
- Properly distinguish between index_root and index_path
3. Dense reranking for centralized search:
- Get chunk metadata from _vectors_meta.db
- Group chunks by source_index_db
- Retrieve dense embeddings from respective _index.db files