Implement deferred L-shaped belt drag placement

src/lib/core/BeltDragPath.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <vector>
+
+#include <QPoint>
+
+#include "Rotation.h"
+
+// One tile of a belt drag-placement path: the tile coordinate and the belt
+// orientation it should be given (REQ-BLD-BELT-DRAG).
+struct BeltPathTile
+{
+    QPoint   tile;
+    Rotation rotation;
+};
+
+// Computes the rectilinear (L-shaped) belt path from `anchor` to `cursor` for a
+// belt whose current orientation is `orientation` (REQ-BLD-BELT-DRAG). The path
+// first runs along the axis parallel to `orientation` (horizontal for East/West,
+// vertical for North/South), stepping toward the cursor's coordinate on that axis
+// to the corner tile, then runs along the orthogonal axis to the cursor tile. Each
+// tile is oriented to point toward the next tile along the path; the final tile
+// keeps the direction of its incoming step, and a single-tile path keeps
+// `orientation`. Returned tiles are ordered from anchor to cursor with no duplicate
+// corner tile.
+std::vector<BeltPathTile> computeBeltDragPath(QPoint anchor, QPoint cursor,
+                                              Rotation orientation);