extract the scroll position into WorldCamera

src/ui/GameWorldView.h

@@ -49,6 +49,7 @@
 #include "TickDriver.h"
 #include "TunnelCompletion.h"
 #include "VisualsConfig.h"
+#include "WorldCamera.h"
 #include "WorldCoordinates.h"
 
 struct Command;
@@ -184,9 +185,9 @@ private:
 
     float    getAsteroidLeftEdge() const;
     float    getEnemyStationRightEdge() const;
-    // Horizontal pan speed at a given view-center X, in tiles/s (REQ-UI-SCROLL-SPEED).
-    float    panSpeedTilesPerSecondAt(float viewCenterXTiles) const;
-    void     clampScroll();
+    // The camera's current pan limits, read fresh from the simulation each frame:
+    // both edges move with asteroid expansion and with pushes (REQ-GW-SCROLL-LIMIT).
+    ScrollBounds getScrollBounds() const;
 
     bool     isValidPlacement(BuildingType type, QPoint anchor, Rotation rot) const;
     std::optional<BuildingId> buildingAtTile(QPoint tile) const;
@@ -333,8 +334,9 @@ private:
     std::mt19937  m_rng;
     double        m_gameSpeedMultiplier;
     double        m_prevNonZeroSpeed;
-    // World-X (tiles) at the center of the viewport (see getViewLeftTiles()).
-    float         m_scrollXTiles;
+    // Horizontal view position (REQ-UI-SCROLL). Owns the scroll position itself;
+    // this widget only supplies the pan intent and the simulation-derived bounds.
+    WorldCamera   m_camera;
 
     QTimer* m_renderTimer;
 
@@ -390,6 +392,11 @@ private:
     QPoint                m_boxStartTile;
     QPoint                m_boxCurrentTile;
 
+    // Held-key state for the A / D pan controls (REQ-UI-SCROLL, REQ-UI-HOTKEYS),
+    // collapsed into a PanDirection for the camera each frame. Kept here rather
+    // than on the camera because it is key state, not view state: once controls
+    // are rebindable this becomes an input mapper publishing the direction, and
+    // the camera's interface does not change.
     bool m_scrollLeft;
     bool m_scrollRight;
     bool m_gameOverShown;