move two placement queries out of the view into PlacementRules

src/lib/sim/PlacementRules.h

@@ -5,6 +5,7 @@
 
 #include <QPoint>
 
+#include "BeltDragPath.h"
 #include "BuildingId.h"
 #include "BuildingType.h"
 #include "FactoryState.h"
@@ -36,3 +37,35 @@ std::optional<BuildingId> findRotateInPlaceTarget(const FactoryState& state,
                                                   const GameConfig& config,
                                                   BuildingType type, QPoint anchor,
                                                   Rotation rot);
+
+// True if placing here would actually do something: the terrain and bounds rules of
+// isPlacementValid hold, and the body cells are either all free or occupied only by
+// a building this placement would rotate in place. This is the question the ghost
+// asks to colour itself and the click path asks before enqueuing a command
+// (REQ-BLD-GHOST, REQ-BLD-PLACE-VALID, REQ-BLD-ROTATE-IN-PLACE).
+bool canPlaceBuilding(const FactoryState& state, const GameConfig& config,
+                      BuildingType type, QPoint anchor, Rotation rotation);
+
+// What a belt drag would do to one tile of its path (REQ-BLD-BELT-DRAG).
+enum class BeltTileAction
+{
+    PlaceNew,       // empty, valid cell: a new belt, subject to affordability
+    RotateInPlace,  // already a belt (or belt site): re-oriented, free
+    Invalid         // occupied by something else, or invalid terrain
+};
+
+struct BeltDragResolved
+{
+    BeltTileAction            action;
+    bool                      affordable;  // meaningful only for PlaceNew
+    std::optional<BuildingId> rotateId;    // set only for RotateInPlace
+};
+
+// Classifies every tile of a belt drag path against the current factory state,
+// spending `buildingBlocksStock` cumulatively across the PlaceNew tiles so a path
+// longer than the player can afford is only partly buildable (REQ-BLD-BELT-DRAG).
+// Shared so the previewed ghosts and the placement on release cannot disagree.
+std::vector<BeltDragResolved> resolveBeltDragPath(const std::vector<BeltPathTile>& path,
+                                                  const FactoryState& state,
+                                                  const GameConfig& config,
+                                                  int buildingBlocksStock);