Add build2 package (cppget.org)

[gistrec]

The library is already available via vcpkg (port geo-utils-cpp) and is in flight for Conan (#17) and xrepo (xmake-io/xmake-repo#9942).
This issue tracks publishing the library to cppget.org to cover the build2 ecosystem.

Status

• Add build2 packaging files (manifest, buildfile, package layout) to the repo
• Install build2 toolchain locally
• Run bdep publish to upload to https://queue.cppget.org
• Package accepted into alpha/beta/testing (auto-promoted on first successful build)
• Promotion to stable (requires human review + tests as separate subpackage)
• Update README with build2 installation instructions

Open questions

• Package name: build2 convention strongly favors lib-prefix and no -cpp suffix → likely libgeo-utils. Trade-off vs. consistency with vcpkg/conan/xrepo where the name is geo-utils-cpp.
• Where to keep packaging files: alongside CMake in master, or in a separate staging repo.

References

• vcpkg port: https://github.com/microsoft/vcpkg/tree/master/ports/geo-utils-cpp
• Conan tracking issue: Add Conan package (conan-center-index) #17
• xrepo PR: Add geo-utils-cpp v1.0.1 xmake-io/xmake-repo#9942
• cppget submission: https://cppget.org/?submit
• bdep-publish docs: https://build2.org/bdep/doc/bdep-publish.xhtml
• build2 packaging guide: https://build2.org/build2-toolchain/doc/build2-toolchain-packaging.xhtml
• HOWTO header-only library: https://github.com/build2/HOWTO/blob/master/entries/make-header-only-library.md