feat(deps): add FindBoost module with components

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>

AGENTS.md

@@ -100,7 +100,7 @@ cppcourse/
 - `Materials/`: supplementary course material PDFs.
 - Each exercise directory holds a `CMakeLists.txt` that defines its target via
   `cmat_add_exercise()` (see `cmat/CMakeLists.txt`). Exercises that need a
-  missing dependency (e.g. Boost) are reported as not available at configure
+  missing dependency (e.g. curses) are reported as not available at configure
   time and skipped.
 
 ## Behavioral Guidelines