feat: add hybrid fusion strategy(dbsf) support for search queries

[srimon12]

Summary

This PR adds support for selecting the hybrid fusion strategy in SEARCH queries.

QQL previously hardcoded hybrid search to Qdrant's RRF fusion mode. With this change, users can now choose either rrf or dbsf explicitly while keeping rrf as the default for backward compatibility.

Example:

SEARCH docs SIMILAR TO 'q' LIMIT 10 USING HYBRID FUSION 'dbsf'

Changes

• add FUSION keyword support to the lexer
• extend hybrid search parsing to accept FUSION 'rrf|dbsf'
• store the selected fusion strategy on SearchStmt
• replace the executor's hardcoded Fusion.RRF with a small strategy resolver
• keep existing behavior unchanged when no fusion is specified
• update CLI help and search/reference docs with the new syntax and examples
• add lexer, parser, and executor tests for dbsf, default rrf, and invalid fusion values

Backward Compatibility

• existing USING HYBRID queries continue to work unchanged
• omitted fusion still defaults to rrf

Testing

• ran pytest -q
• result: 405 passed