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.cpp

@@ -0,0 +1,59 @@
+#include "Board.h"
+#include <algorithm>
+
+namespace logic {
+
+Board::Board()
+    : grid_(Width * Height, Cell::Empty)
+{
+}
+
+void Board::clear()
+{
+    std::fill(grid_.begin(), grid_.end(), Cell::Empty);
+}
+
+bool Board::inBounds(int x, int y) const
+{
+    return x >= 0 && x < Width && y >= 0 && y < Height;
+}
+
+Board::Cell Board::at(int x, int y) const
+{
+    if (!inBounds(x, y)) return Cell::Empty;
+    return grid_[y * Width + x];
+}
+
+void Board::set(int x, int y, Cell c)
+{
+    if (!inBounds(x, y)) return;
+    grid_[y * Width + x] = c;
+}
+
+int Board::clearFullLines()
+{
+    int cleared = 0;
+    // scan from bottom to top
+    for (int y = Height - 1; y >= 0; --y) {
+        bool full = true;
+        for (int x = 0; x < Width; ++x) {
+            if (at(x, y) == Cell::Empty) { full = false; break; }
+        }
+        if (full) {
+            // remove row y: move all rows above down by one
+            for (int yy = y; yy > 0; --yy) {
+                for (int x = 0; x < Width; ++x) {
+                    grid_[yy * Width + x] = grid_[(yy - 1) * Width + x];
+                }
+            }
+            // clear top row
+            for (int x = 0; x < Width; ++x) grid_[x] = Cell::Empty;
+            ++cleared;
+            // stay on same y to re-check the row that fell into place
+            ++y; // because next iteration decrements y
+        }
+    }
+    return cleared;
+}
+
+} // namespace logic