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_DOS.cpp

@@ -0,0 +1,32 @@
+#include <conio.h>
+#include <string>
+#include <windows.h>
+using std::string;
+
+#include "Screen.h"
+
+namespace {
+HANDLE hStdout = 0;
+}
+
+void Screen::write(string s, int x, int y, int color) {
+    if (hStdout == 0) {
+        hStdout = GetStdHandle(STD_OUTPUT_HANDLE);
+    }
+
+    COORD screenPos;
+    screenPos.X = x;
+    screenPos.Y = y;
+
+    unsigned long ln;
+
+    static unsigned short attrib[80];
+
+    for (unsigned int i = 0; i < s.length(); ++i) {
+        attrib[i] = color;
+    }
+
+    WriteConsoleOutputAttribute(hStdout, attrib, s.length(), screenPos, &ln);
+
+    WriteConsoleOutputCharacter(hStdout, s.c_str(), s.length(), screenPos, &ln);
+}