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>

cmat/CMakeLists.txt

@@ -56,10 +56,8 @@ else()
     # Windows-specific; there is nothing portable to build with.
 endif()
 
-# Exercises using Boost.Serialization (system install; not fetched). CONFIG
-# mode uses Boost's own CMake package, since the FindBoost module was removed
-# in CMake 4.
-find_package(Boost CONFIG QUIET COMPONENTS serialization)
+# Exercises using Boost.Serialization (fetched via deps/FindBoost.cmake).
+find_package(Boost QUIET COMPONENTS serialization)
 
 # ------------------------------------------------------------------------------
 # Target helpers

cmat/SolutionEx62Bonus/Employee.h

@@ -2,6 +2,8 @@
 #define EMPLYOEE_H
 #include "Person.h"
 
+#include <boost/serialization/base_object.hpp>
+
 class Employee : public Person {
   public:
     void askForInfo();