[M2] Multi-threaded indexing with Rayon

[Fosurero]

Description

Parallelize index building using Rayon for multi-core utilization.

Tasks

• Add rayon dependency
• Shard-parallel indexing (one thread per shard)
• Thread-safe index merging
• Benchmark: single-threaded vs multi-threaded index build
• Configurable thread count

Acceptance Criteria

• Index build is ≥2× faster on 4-core machine
• Results are identical to single-threaded mode