Convert hamming.cpp public API to dynamic dispatch (#5070)

[algoriddle]

Summary:

Convert all 8 public Hamming distance functions in hamming.cpp to
dynamic dispatch so they use the correct SIMD implementation at runtime
instead of always running at scalar speed in DD mode.

This is Phase 1 of the Hamming DD conversion. It converts the public API
functions in hamming.cpp; Phase 2 (future diff) will migrate external
callers that use dispatch_HammingComputer directly.

Why Hamming is harder than previous DD conversions:
The SIMD code lives inside 11 HammingComputer/GenHammingComputer structs
with ISA-specific memory layouts (e.g., NEON HammingComputer16 stores
uint8x16_t a0 vs x86 uint64_t a0, a1). The structs use raw
intrinsics (not simdlib) and are template parameters in hot inner loops
where .hamming() must be inlined.

Approach:

• Extract all template code from hamming.cpp into a shared header
  hamming_impl.h, compiled once per ISA TU (AVX2, NEON, NONE).
• The hamdis-inl.h ISA ladder (#ifdef __AVX2__ etc.) selects the
  correct struct definitions based on per-TU compiler flags.
• All internal templates live in an anonymous namespace for ODR safety
  (different TUs see different struct layouts but never share instances).
• 8 entry point template specializations (_dispatch<SL>) provide the
  external linkage symbols that the dispatch wrappers call.
• hamming.cpp includes hamming_impl.h with THE_SIMD_LEVEL=NONE
  for the scalar fallback, then dispatches via with_simd_level_256bit.
• hamming.h keeps including hamdis-inl.h for backward compatibility
  with external callers (they continue to work with generic structs).

Functions dispatched:

• hammings_knn_hc (binary flat/IVF kNN search)
• hammings_knn_mc (max-count kNN)
• hamming_range_search
• hammings (all-pairs distance)
• generalized_hammings_knn_hc (byte-level Hamming)
• hamming_count_thres, crosshamming_count_thres, match_hamming_thres

Performance impact:

• NEON: HammingComputer16/20/32/64 gain real vectorization (vcntq_u8)
• AVX2: GenHammingComputer16/32/M8 gain SSE2/AVX2 byte comparison
• AVX2: All HammingComputers get hardware popcnt (implied by -mpopcnt)

Differential Revision: D99994593

[meta-codesync]

@algoriddle has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99994593.