sanitize bf16 options on model loading

[nihui]

No description provided.

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[Copilot]

Pull request overview

This PR adds sanitization checks for bf16 (bfloat16) options during model loading to ensure they are disabled if the Vulkan device doesn't support them. This prevents potential runtime errors or undefined behavior when bf16 features are enabled but not supported by the GPU hardware.

Changes:

• Added bf16 packed and storage option sanitization checks in Net::load_param()
• Added bf16 packed and storage option sanitization checks in Net::load_param_bin()

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.74%. Comparing base (6582aeb) to head (de28c24).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6522      +/-   ##
==========================================
- Coverage   93.08%   92.74%   -0.35%     
==========================================
  Files         809      809              
  Lines      256677   255510    -1167     
==========================================
- Hits       238934   236960    -1974     
- Misses      17743    18550     +807     

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