[Enhance]: Support the Flat index compute distances through turbo Quantizer

[richyreachy]

Affected Component

Flat Searcher, Flat Builder, Flat Streamer

Current Behavior

The turbo module already provides registered quantizers (Fp32Quantizer, Fp16Quantizer,
Int8Quantizer, Int4Quantizer, PqInt8Quantizer) with own SIMD distance kernels and APIs.

Every distance in the Flat path goes through FlatDistanceMatrix / IndexMetric
(distance_matrix().distance<1>(...), row_distance_, column_distance_). Quantized data can
therefore only be searched by going through a Converter/Reformer pair and a matching IndexMetric,
which duplicates logic that the turbo quantizers already implement (and better).

Desired Improvement

Plumb an optional std::shared_ptr<zvec::turbo::Quantizer> through the framework and make the Flat
index delegate distance computation to it when present.

Impact

• Turbo quantizers become usable from the real Flat build/search flow instead of tests only, so
  int8/int4/fp16 indexes get the turbo SIMD kernels (AVX2 / AVX‑512 / AVX‑512‑VNNI with scalar
  fallback) rather than a Converter + generic IndexMetric chain.
• The quantizer choice is persisted in the index meta, so a quantized index is self-describing and
  the query side is reconstructed automatically at load time.
• Zero behavior change for existing indexes: without quantizer_name in the meta, every code path
  falls back to the current metric/distance-matrix logic.