fix issue where construction sites could be placed outside of game world and add tests

src/lib/sim/BuildingSystem.h

@@ -40,11 +40,23 @@ public:
                    std::mt19937& rng);
 
     // -- Placement / demolish ------------------------------------------------
-    // Returns the new entity id.  Belt and Splitter register with BeltSystem
-    // directly; other types enter the construction queue.
+    // Returns the new entity id, or kInvalidBuildingId if the placement falls
+    // outside the world bounds (vertical extent and asteroid left edge).  Belt
+    // and Splitter register with BeltSystem directly; other types enter the
+    // construction queue.  Terrain type (A vs S) is NOT checked here so that
+    // tests can stage arbitrary layouts; the player-facing entry point
+    // (Simulation::tryPlaceBuilding) enforces the full rule via isPlacementValid.
     BuildingId place(BuildingType type, QPoint anchor, Rotation rotation,
                    Tick currentTick);
 
+    // Returns true if the placement satisfies REQ-BLD-PLACE-VALID terrain and
+    // world-bounds rules: every ship-dock (S) cell sits in space (x >= 0), every
+    // other body (A) cell sits on the asteroid (x < 0 and x >= the left edge),
+    // and every cell has 0 <= y < world.height_tiles. There is no right-side
+    // bound — space extends rightward. Tile occupancy is NOT checked here.
+    bool isPlacementValid(BuildingType type, QPoint anchor,
+                          Rotation rotation) const;
+
     // Remove a building or construction site by id.  Returns the refund in
     // building blocks (floor(cost * refundPercentage / 100)).  Returns 0 for
     // unknown ids.
@@ -136,6 +148,9 @@ private:
     void                initShipyardBuffers(Building& b) const;
     std::vector<Port>   computeInputPorts(const Building& b) const;
     std::vector<Item>   rollReprocessingOutput(const RecipeDef& recipe);
+    bool                bodyCellsWithinWorldBounds(
+                            const std::vector<QPoint>& bodyCells,
+                            QPoint anchor) const;
 
     const GameConfig&                              m_config;
     BeltSystem&                                    m_belts;