Fetch the Boost CMake release archive (default 1.92.0) and build only
the libraries requested via find_package(Boost COMPONENTS ...), plus
their dependencies. Each component is exposed as Boost::<name>, e.g.
Boost::serialization. cmat now uses this module instead of a system
Boost install, so the Boost exercises are always available.
Also fix SolutionEx62Bonus: Employee.h used base_object without
including <boost/serialization/base_object.hpp>. This was not an
intentional exercise bug; it likely relied on a transitive include
in older Boost versions.
Co-authored-by: Claude Opus 5.5 <claude.opus5.5@anthropic.com>
