x86: add BinaryOp_x86 bf16s storage support with avx512bf16 dispatch

[nihui]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.63031% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.83%. Comparing base (67a20c2) to head (f9b82ad).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/layer/x86/binaryop_bf16s.h	99.43%	1 Missing ⚠️
src/layer/x86/binaryop_x86.cpp	99.62%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6591      +/-   ##
==========================================
+ Coverage   92.71%   92.83%   +0.11%     
==========================================
  Files         846      849       +3     
  Lines      267472   268049     +577     
==========================================
+ Hits       247980   248834     +854     
+ Misses      19492    19215     -277     

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[Copilot]

Pull request overview

Adds BF16 (bf16s) storage support to the x86 BinaryOp layer, including an AVX512-BF16 optimized implementation with runtime CPU dispatch.

Changes:

• Refactors BinaryOp SIMD functors into a shared header (binaryop_x86_functor.h) for reuse across translation units.
• Adds bf16s compute paths for BinaryOp (including broadcasting) and wires them into BinaryOp_x86 forward/forward_inplace.
• Introduces an AVX512BF16-dispatched bf16s vector kernel implementation (binaryop_x86_avx512bf16.cpp).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/layer/x86/binaryop_x86_functor.h	New shared SIMD functor definitions for BinaryOp operations.
src/layer/x86/binaryop_x86_bf16s.h	New bf16s vector/broadcast kernels used by BinaryOp bf16 storage path.
src/layer/x86/binaryop_x86_avx512bf16.cpp	AVX512BF16-optimized bf16s vector kernel entrypoint for runtime dispatch.
src/layer/x86/binaryop_x86.h	Declares bf16s forward helpers on BinaryOp_x86 (guarded by NCNN_BF16).
src/layer/x86/binaryop_x86.cpp	Wires bf16s support, functor refactor, and AVX512BF16 runtime dispatch into x86 BinaryOp.

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