cmake/ci: enforce install contract for packagers (fail-fast + smoke test)

[piotrekoo]

Fixes #450,

This PR hardens the install contract for downstream packagers (e.g. Homebrew) by validating installation end-to-end instead of relying on manual downstream workarounds.

What changed

• Keep canonical install rules for:
  • install(TARGETS lib3mf ...)
  • lib3mf.pc
• Add fail-fast post-build verification that the built lib3mf target file exists:
  • cmake/VerifyTargetFile.cmake
  • invoked via POST_BUILD in CMake.
• Add packager-oriented install smoke test:
  • CI/install_smoke_test.sh
  • configures with:
    • -DUSE_INCLUDED_ZLIB=OFF
    • -DUSE_INCLUDED_LIBZIP=OFF
    • -DUSE_INCLUDED_SSL=OFF
    • -DLIB3MF_TESTS=OFF
  • runs cmake --install
  • verifies installed artifacts:
    • lib3mf*.so|dylib
    • lib3mf.pc
    • C headers (lib3mf.h, lib3mf_types.h)
  • verifies downstream usability via:
    • pkg-config --cflags --libs lib3mf
    • minimal compile/link test with #include <lib3mf.h>.
• Add CI job install-linux-packagers-smoke to enforce this contract in upstream CI.
• Updated install-dir handling to respect GNUInstallDirs defaults unless explicitly overridden by the caller.
• Updated lib3mf.pc template:
  • moved transitive deps to Libs.private (-lzip -lz)
  • updated Cflags to point to the C API headers in Bindings/C.

Why

Install regressions can reappear if only install rules exist but are not validated after cmake --install.
This PR makes the packager configuration explicitly tested and guaranteed upstream.