[goroyabu]

Summary

• add an isolated install test that configures with an unused prefix, installs
  directly to an override prefix, and removes disposable source and build inputs
• verify the installed CMake package, imported artifacts, include directories,
  and consumer behavior resolve exclusively from the installed prefix
• run the workflow in a dedicated Ubuntu 24.04 GCC CI job with deletion-safety
  guards around its temporary test root

Related Issue

Closes #64

Verification

• isolated install workflow completed successfully on macOS with AppleClang
• safety checks rejected a source-contained test root, an invalid root name,
  and an existing unowned root while preserving its sentinel file
• clean offline configure and build completed successfully
• full local test suite passed: 68/68 tests
• git diff --check passed
• the dedicated Ubuntu 24.04 job could not be run locally and is expected to
  provide the Linux-specific verification in this pull request

Impact

No installed interface or documented user workflow changes. The pull request
adds automated verification that the existing install-time prefix override
produces a self-contained CMake package independent of disposable build inputs.
Arbitrary movement of an already-installed prefix remains outside this test's
scope.

Documentation

No documentation changes. The tested workflow is already documented, and this
change adds executable verification without changing its public contract.

Checklist

• The change is focused on one coherent maintenance outcome.
• Relevant local verification has been run and reported above.
• User-facing workflows and supported behavior are documented when changed.
• No downloaded upstream archives, extracted vendor sources, build outputs,
  or generated files were committed unintentionally.