Implement exact/baseline multi-vector distance

Description

Before introducing highly optimized, vectorized, or quantized microkernels, we need a ground-truth implementation of the multi-vector distance functions (focusing initially on Chamfer distance). This will act as the baseline exact-match implementation and validate the standalone API.

Tasks

• Implement a straightforward, unoptimized Chamfer distance calculation (f32).
• Expose this via the new standalone distance API.
• Implement basic ColBERT/MaxSim fallback functionality.

Note: Unit tests for all future AVX2/SIMD microkernels and quantized implementations will be asserted against the outputs of this baseline to prevent precision regressions.