feat(cmat): add cmake build for course material

Add a CMakeLists.txt to each exercise directory, included from
cmat/CMakeLists.txt, so every exercise and solution becomes a buildable
target. GUI exercises link the curses-based DOS_Unix implementations on
Unix-like systems; the Windows-specific DOS variants are not compiled.
Exercises without a portable path or with missing dependencies (Boost)
are reported as not available at configure time.

Targets are excluded from the default build and built by name, e.g.
`cmake --build build --target Ex11`, so intentionally buggy exercises
such as Ex21 do not break the default build.

Also fix two portability issues: rename Ex11bug/pole.h to Pole.h for
case-sensitive filesystems and drop the extra Light:: qualification in
SolutionEx71A/Light.h. Document build commands and layout in AGENTS.md.

Co-authored-by: Qwen 3.8 27B <qwen3.8_27b-q3_k_xl@lmstudio.local>

CMakeLists.txt

@@ -22,6 +22,9 @@ target_compile_options(hello PRIVATE ${BASE_OPTIONS})
 target_compile_definitions(hello PRIVATE ${BASE_DEFINITIONS})
 target_link_libraries(hello PRIVATE fmt::fmt)
 
+# Course material (exercises and solutions)
+add_subdirectory(cmat)
+
 option(BUILD_TESTING OFF "Build Unity/CMock test suites")
 if (BUILD_TESTING AND NOT IS_EMSCRIPTEN)
     set(CMAKE_SKIP_TEST_ALL_DEPENDENCY FALSE)