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>

cmat/DOS_Unix/Screen.h

@@ -0,0 +1,19 @@
+#ifndef SCREEN_H
+#define SCREEN_H
+
+#include <string>
+
+namespace Screen {
+const int BLACK = 0;
+const int GRAY = 8;   // FOREGROUND_INTENSITY;
+const int BLUE = 9;   // FOREGROUND_RED|FOREGROUND_INTENSITY
+const int GREEN = 10; // FOREGROUND_GREEN|FOREGROUND_INTENSITY;
+const int RED = 12;   // FOREGROUND_RED|FOREGROUND_INTENSITY
+const int YELLOW = RED | GREEN;
+const int WHITE = RED | GREEN | BLUE;
+const int BACKGROUND = 128; // OR (|) with the color value for gray background
+
+void write(std::string s, int x, int y, int color);
+};
+
+#endif