AVX512, default Bulk SIMD for Faiss Scala Quantized Index.

[0ctopus13prime]

Description

1. Fix: Undefined behavior in int4BitDotProduct scalar fallback
   The int4BitDotProduct function in both avx512_simd_similarity_function.cpp and arm_neon_simd_similarity_function.cpp used reinterpret_cast<const uint64_t*> on pointers that may not be 8-byte aligned when binaryCodeBytes % 8 != 0. This is undefined behavior per the C++ standard.

Replaced the casts with std::memcpy into local uint64_t variables. Compilers optimize this into a single mov — zero runtime cost, no UB.

2. Feature: BBQ similarity support for the default (non-SIMD) fallback
   The default similarity function (default_simd_similarity_function.cpp) only supported FP16. BBQ queries on platforms without AVX512 or NEON would fail with "Invalid native similarity function type".

Added the full BBQ scoring pipeline to the default implementation:

readDataCorrections — safe unaligned reads via std::memcpy
int4BitDotProduct — scalar weighted popcount dot product (with the alignment fix)
default4bitDotProductBatch<BATCH_SIZE> — batched version, pure C++, no SIMD intrinsics
DefaultBBQSimilarityFunction — full scoring struct with batch-8/batch-4/scalar-tail pattern
Wired up BBQ_IP and BBQ_L2 in selectSimilarityFunction

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• [O] New functionality includes testing.
• [O] New functionality has been documented.
• [O] API changes companion pull request created.
• [O] Commits are signed per the DCO using --signoff.

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