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).

tests/test_board.cpp

@@ -0,0 +1,31 @@
+#include "../src/logic/Board.h"
+#include <cassert>
+#include <iostream>
+
+int main()
+{
+    using logic::Board;
+    Board b;
+    // board starts empty
+    for (int y = 0; y < Board::Height; ++y)
+        for (int x = 0; x < Board::Width; ++x)
+            assert(b.at(x,y) == Board::Cell::Empty);
+
+    // fill a single full row at bottom
+    int y = Board::Height - 1;
+    for (int x = 0; x < Board::Width; ++x) b.set(x, y, Board::Cell::Filled);
+    int cleared = b.clearFullLines();
+    assert(cleared == 1);
+    // bottom row should be empty after clear
+    for (int x = 0; x < Board::Width; ++x) assert(b.at(x, Board::Height - 1) == Board::Cell::Empty);
+
+    // fill two non-adjacent rows and verify both clear
+    int y1 = Board::Height - 1;
+    int y2 = Board::Height - 3;
+    for (int x = 0; x < Board::Width; ++x) { b.set(x, y1, Board::Cell::Filled); b.set(x, y2, Board::Cell::Filled); }
+    cleared = b.clearFullLines();
+    assert(cleared == 2);
+
+    std::cout << "tests/test_board: all assertions passed\n";
+    return 0;
+}