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

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();

deps/FindBoost.cmake

@@ -0,0 +1,89 @@
+# ==============================================================================
+# Find Boost
+# ==============================================================================
+# This module fetches the Boost libraries (CMake release archive) and builds
+# only the requested components and their dependencies.
+#
+# Usage:
+#   find_package(Boost REQUIRED COMPONENTS serialization program_options)
+#   target_link_libraries(app PRIVATE Boost::serialization Boost::program_options)
+#
+# Components are Boost library names as they appear under libs/ (lowercase),
+# e.g. serialization, filesystem, json, asio. Header-only libraries work too.
+# Without components every Boost library is configured (slower configure);
+# targets are only built when linked.
+#
+# All components must be requested in the first find_package(Boost) call:
+# Boost is configured once, later calls can only use what was configured.
+#
+# Targets provided:
+#   Boost::<component> - One target per requested component
+#   Boost::headers     - All configured header-only libraries
+#
+# Variables set:
+#   Boost_FOUND             - TRUE if Boost and all required components are available
+#   Boost_<component>_FOUND - TRUE if the component is available
+#   Boost_LIBRARIES         - The component targets (Boost::<component>...)
+#   Boost_VERSION           - Version of Boost
+# ==============================================================================
+
+# Use the version passed to find_package(), or default to 1.92.0
+# The version must be the full x.y.z form used by the release archive names
+if (DEFINED Boost_FIND_VERSION AND NOT Boost_FIND_VERSION STREQUAL "")
+    set(BOOST_VERSION "${Boost_FIND_VERSION}")
+else()
+    set(BOOST_VERSION "1.92.0")
+endif()
+
+if (NOT DEFINED _FINDBOOST_INCLUDED)
+    set(_FINDBOOST_INCLUDED TRUE)
+
+    message(STATUS "Fetching Boost ${BOOST_VERSION} (components: ${Boost_FIND_COMPONENTS})")
+
+    include(FetchContent)
+
+    # Mark Boost includes as SYSTEM to suppress warnings from its headers
+    set(_boost_fetch_options "")
+    if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.25)
+        list(APPEND _boost_fetch_options SYSTEM)
+    endif()
+
+    FetchContent_Declare(
+        boost
+        URL https://github.com/boostorg/boost/releases/download/boost-${BOOST_VERSION}/boost-${BOOST_VERSION}-cmake.tar.xz
+        DOWNLOAD_EXTRACT_TIMESTAMP TRUE
+        EXCLUDE_FROM_ALL
+        ${_boost_fetch_options}
+    )
+
+    # Read by the Boost superproject: build only these libraries (and their
+    # dependencies). Empty means all libraries.
+    set(BOOST_INCLUDE_LIBRARIES ${Boost_FIND_COMPONENTS})
+    FetchContent_MakeAvailable(boost)
+    unset(BOOST_INCLUDE_LIBRARIES)
+
+    set(BOOST_LICENSE_FILE "${boost_SOURCE_DIR}/LICENSE_1_0.txt" CACHE FILEPATH "Path to Boost license file")
+endif()
+
+set(Boost_LIBRARIES "")
+foreach (_boost_comp IN LISTS Boost_FIND_COMPONENTS)
+    if (TARGET Boost::${_boost_comp})
+        set(Boost_${_boost_comp}_FOUND TRUE)
+        list(APPEND Boost_LIBRARIES Boost::${_boost_comp})
+    else()
+        set(Boost_${_boost_comp}_FOUND FALSE)
+        if (NOT Boost_FIND_QUIETLY)
+            message(STATUS "Boost component '${_boost_comp}' is not available: "
+                "unknown library, or not requested in the first find_package(Boost) call")
+        endif()
+    endif()
+endforeach()
+
+set(Boost_VERSION "${BOOST_VERSION}")
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Boost
+    REQUIRED_VARS boost_SOURCE_DIR
+    VERSION_VAR   Boost_VERSION
+    HANDLE_COMPONENTS
+)