Update webp to 1.6.0

[iamcarbon]

Summary

• Update the project-specific libwebp port from 1.4.0 to 1.6.0.
• Refresh the libwebp source checksum.
• Update the SIMD patch to match the vcpkg 2026.07.29 source layout and compiler flags.
• Update the managed WebP runtime version guard and display label to 1.6.0.

Testing

• Successfully built and installed the custom libwebp[core,libwebpmux,nearlossless,simd,unicode]:x64-windows@1.6.0 port.
• Verified all 140 managed WebP interop declarations exist in the installed libwebp 1.6.0 headers.