Switch flat + IVF search to FastScanCodeScanner dispatch

[algoriddle]

Summary:
Switch the actual search paths in IndexFastScan and IndexIVFFastScan to
use the SIMD-dispatched FastScanCodeScanner instead of the old
make_knn_handler() + free-function accumulate pattern.

Each index gains a virtual make_knn_scanner() that returns a
FastScanCodeScanner (bundling handler + kernel behind the SIMD dispatch
boundary). Every search function follows the same pattern: try
make_knn_scanner() first; if it returns nullptr (RaBitQ with custom
handlers), fall back to the existing make_knn_handler() path.

Flat index (IndexFastScan):

• search_implem_12 (QBS path): scanner->accumulate_loop_qbs(), fallback
• search_implem_14 (multi-BB path): scanner->accumulate_loop(), fallback

IVF index (IndexIVFFastScan):

• search_dispatch_implem: try scanner in both single-thread and
  multi-thread paths, pass to implem_10/12
• search_implem_10: accept optional scanner, use scanner->accumulate_loop()
• search_implem_12: accept optional scanner, use scanner->accumulate_loop_qbs()
• search_implem_14: create per-thread scanner, use if available

IVFPQFastScan scanner (IVFPQFastScanScanner::scan_codes):

• Try scanner->accumulate_loop(), fallback to handler

RaBitQ overrides (IndexRaBitQFastScan, IndexIVFRaBitQFastScan):

• make_knn_scanner() returns nullptr (scanner wiring comes in next diff)

SWIG: ignore FastScanCodeScanner, make_fast_scan_knn_scanner,
make_fast_scan_scanner_impl, all make_knn_scanner overrides, and
search_implem_10/12 (which now take FastScanCodeScanner* parameter).

Differential Revision: D95950481

[meta-codesync]

@algoriddle has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95950481.