fix windows-arm build

[nihui]

No description provided.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.95%. Comparing base (d0d5063) to head (1151f9e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6699      +/-   ##
==========================================
- Coverage   93.95%   93.95%   -0.01%     
==========================================
  Files         933      933              
  Lines      299730   299948     +218     
==========================================
+ Hits       281624   281816     +192     
- Misses      18106    18132      +26     

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[Copilot]

Pull request overview

This PR targets Windows ARM64 (MSVC) build issues in the ARM FP16 NEON path by centralizing MSVC-specific FP16 scalar/constant handling in arm_usability.h, and then simplifying call sites to use the unified helpers.

Changes:

• Added MSVC ARM64 float32↔float16 bit-conversion helpers and vdup*_n_f16(float) shims in arm_usability.h to avoid relying on scalar __fp16 and problematic constant materialization patterns.
• Simplified PReLU_arm::forward_inplace_fp16sa() (elempack==4, dims==1 path) to remove MSVC-only branches and use the unified vdup_n_f16/vcle_f16 usage.
• Removed MSVC-specific constant conversion branches in neon_mathfun_fp16s.h, relying on the unified constant construction behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/layer/arm/prelu_arm_asimdhp.cpp	Removes MSVC-only FP16 NEON special-casing in the pack4/dims1 PReLU FP16SA path.
src/layer/arm/neon_mathfun_fp16s.h	Drops MSVC-only constant materialization branches in FP16 exp/sincos helpers.
src/layer/arm/arm_usability.h	Introduces MSVC ARM64 FP16 conversion + vdup*_n_f16(float) shims to support the simplified call sites.

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