[#111]:amelia:github, enable macOS CI runner (macos-13, clang-17/18, Homebrew HDF5)

[steven-varga]

Closes #111

Summary

Adds macos-13 to the CI matrix. Active macOS combinations: clang-17 and clang-18 via Homebrew LLVM. GCC and clang-19/20 are skipped (gray badges).

Changes:

• ci.yml: add macos-13 to os:; activate existing dead macos-13:gcc-* skip entries; add macos-13:clang-19 and macos-13:clang-20 to skip list; add -DH5CPP_BUILD_TESTS=ON to Configure step
• CMakeLists.txt: replace ***WARNING: h5cpp is not fully tested on apple*** with an informational message now that CI is active

Effective matrix on macOS:

	gcc-13	gcc-14	gcc-15	clang-17	clang-18	clang-19	clang-20
macos-13	○	○	○	runs	runs	○	○

Test plan

• CI green on this PR for macos-13 / clang-17 and macos-13 / clang-18
• Linux matrix unaffected

🤖 Generated with Claude Code

[steven-varga]

On hold. GitHub Actions macOS runners are not a reliable CI substrate for this project — queue times are unpredictable and the platform is not stable enough for blocking pre-merge checks. This PR will not be merged until a stable macOS testing platform is identified (candidates: Cirrus CI, self-hosted runner). Linux CI coverage is sufficient to proceed in the interim.