Add FastScanCodeScanner dispatch boundary with per-SIMD TUs

Summary:
Add `FastScanCodeScanner`, a virtual base that bundles handler + kernel
behind the SIMD dispatch boundary. In DD mode, `SINGLE_SIMD_LEVEL = NONE`
so the existing fast scan code path uses emulated SIMD types. The new
scanner provides per-SIMD translation units (AVX2, AVX512, ARM_NEON)
compiled with the correct ISA flags, and a factory function
(`make_fast_scan_knn_scanner`) that uses `DISPATCH_SIMDLevel` to select
the right TU at runtime.

This follows the proven `THE_LEVEL_TO_DISPATCH` pattern from the scalar
quantizer per-SIMD TUs (`sq-dispatch.h`). Each per-SIMD TU includes
`dispatching.h` which provides:
- `ScannerMixIn<Handler>`: wraps a concrete handler and calls accumulation
  kernels (both search_1 multi-BB and QBS paths)
- Factory specialization `make_fast_scan_scanner_impl<SL>()` with
  combinatorial dispatch over `is_max × with_id_map × handler_type`
  (SingleResultHandler for k=1, HeapHandler for k≤20, ReservoirHandler
  for k>20)

New files:
- `impl/fast_scan/dispatching.h` — dispatch template header
- `impl/fast_scan/impl-avx2.cpp` — AVX2 per-SIMD TU
- `impl/fast_scan/impl-avx512.cpp` — AVX512 per-SIMD TU
- `impl/fast_scan/impl-neon.cpp` — ARM NEON TU (with ARM_SVE forwarding)

Modified files:
- `impl/fast_scan/pq4_fast_scan.h` — FastScanCodeScanner base + factory decl
- `impl/fast_scan/pq4_fast_scan.cpp` — NONE specialization + dispatch wrapper
- `xplat.bzl` / `CMakeLists.txt` — register SIMD files and header

Note: RaBitQ handler is not wired through FastScanCodeScanner in this
diff. That comes in later diffs when callers are switched.

Differential Revision: D95950483

Author: algoriddle

faiss/CMakeLists.txt

@@ -9,16 +9,19 @@
 # Architecture-specific: only include files for the current build architecture
 # =============================================================================
 set(FAISS_SIMD_AVX2_SRC
+  impl/fast_scan/impl-avx2.cpp
   impl/pq_code_distance/pq_code_distance-avx2.cpp
   impl/scalar_quantizer/sq-avx2.cpp
   utils/simd_impl/distances_avx2.cpp
 )
 set(FAISS_SIMD_AVX512_SRC
+  impl/fast_scan/impl-avx512.cpp
   impl/pq_code_distance/pq_code_distance-avx512.cpp
   impl/scalar_quantizer/sq-avx512.cpp
   utils/simd_impl/distances_avx512.cpp
 )
 set(FAISS_SIMD_NEON_SRC
+  impl/fast_scan/impl-neon.cpp
   impl/scalar_quantizer/sq-neon.cpp
   utils/simd_impl/distances_aarch64.cpp
 )
@@ -262,6 +265,7 @@ set(FAISS_HEADERS
   impl/kmeans1d.h
   impl/lattice_Zn.h
   impl/platform_macros.h
+  impl/fast_scan/dispatching.h
   impl/fast_scan/pq4_fast_scan.h
   impl/fast_scan/decompose_qbs.h
   impl/fast_scan/kernels_simd256.h