feat(cmake): support find_package(libhv)

[ithewei]

Summary

• generate versioned libhv CMake package files from a config template
• export namespaced targets libhv::hv and libhv::hv_static while keeping compatibility variables/targets
• add a small external find_package smoke-test fixture and document downstream usage

Verification

• cmake -S . -B /private/tmp/libhv-find-package-delete-check/build -DCMAKE_INSTALL_PREFIX=/private/tmp/libhv-find-package-delete-check/install -DBUILD_EXAMPLES=OFF -DBUILD_UNITTEST=OFF -DBUILD_SHARED=ON -DBUILD_STATIC=ON
• cmake --build /private/tmp/libhv-find-package-delete-check/build --target install --config Release
• cmake -S tests/cmake/find_package -B /private/tmp/libhv-find-package-delete-check/consumer-build -DCMAKE_PREFIX_PATH=/private/tmp/libhv-find-package-delete-check/install
• cmake --build /private/tmp/libhv-find-package-delete-check/consumer-build --config Release

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