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/Ex33/Pixel.h

@@ -0,0 +1,42 @@
+
+#ifndef Pixel_h
+#define Pixel_h
+
+#include <functional>
+#include <iostream>
+
+class Pixel {
+  public:
+    Pixel(int x = 0, int y = 0) : x(x), y(y) {}
+
+    void print() const { std::cout << '(' << x << "," << y << ") "; }
+
+    bool operator<(const Pixel &rhs) const { return x < rhs.x; }
+
+    bool operator==(const Pixel &rhs) const { return x == rhs.x; }
+
+  private:
+    int x;
+    int y;
+
+    friend class PixelPtrCompare;
+};
+
+class PixelPtrCompare {
+  public:
+    PixelPtrCompare() : sortOnY(false) {}
+
+    void setSortOnY(bool sortY) { this->sortOnY = sortY; }
+
+    bool operator()(const Pixel *l, const Pixel *r) const {
+        if (sortOnY)
+            return l->y < r->y;
+        else
+            return l->x < r->x;
+    }
+
+  private:
+    bool sortOnY;
+};
+
+#endif