Add MDT posting scan and approximate top-K candidate generation

[chrevanthreddy]

Parent: #19094
Related: #18676

Summary

Add MDT posting scan and approximate top-K candidate generation.

Scope

• posting scan over selected cluster ranges
• approximate scoring
• multibit-aware candidate refinement where applicable
• shortlist generation for downstream rerank

Acceptance Criteria

• approximate candidate generation is implemented independently of exact rerank
• multibit-aware behavior is part of the MVP candidate path
• top-K shortlist semantics are tested