Add native model manager build script flags

[0xShug0]

Summary

• add first-class native model manager flags to Linux, macOS, and Windows build scripts
• make Windows prebuilt packaging opt into native model manager builds so the packaged manager binary is produced reliably
• document the script flags for native WebUI model downloads and TLS backend selection

Validation

• git diff --check
• bash -n scripts/build_linux.sh scripts/build_metal.sh scripts/build_xcframework.sh
• ./scripts/build_linux.sh --backend cpu --build-dir build/debug --build-type Debug --native-model-manager --target audiocpp_server --target audiocpp_model_manager --jobs $(nproc)
• verified AUDIOCPP_BUILD_NATIVE_MODEL_MANAGER=ON and AUDIOCPP_USE_SYSTEM_OPENSSL=OFF in CMake cache
• verified produced server and model-manager binaries run with --help
• verified no dynamic ssl/crypto/curl dependency in the produced Linux binaries

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