Scaffold the pure game model

- Added a pure, SDL-free Board model implementing grid access and clearFullLines().
- Added a small standalone test at test_board.cpp (simple assert-based; not yet wired into CMake).

src/logic/Board.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <vector>
+#include <cstdint>
+
+namespace logic {
+
+class Board {
+public:
+    static constexpr int Width = 10;
+    static constexpr int Height = 20;
+
+    enum class Cell : uint8_t { Empty = 0, Filled = 1 };
+
+    Board();
+
+    void clear();
+
+    Cell at(int x, int y) const;
+    void set(int x, int y, Cell c);
+    bool inBounds(int x, int y) const;
+
+    // Remove and return number of full lines cleared. Rows above fall down.
+    int clearFullLines();
+
+    const std::vector<Cell>& data() const { return grid_; }
+
+private:
+    std::vector<Cell> grid_; // row-major: y*Width + x
+};
+
+} // namespace logic