Unify tunnel build mode into a single Tunnel button

src/ui/GameWorldView.h

@@ -205,6 +205,16 @@ private:
     void                     stepSpeed(int delta);
     void                     placeAtTile(QPoint tile);
 
+    // Unified tunnel build mode (REQ-BLD-TUNNEL-MODE). Active while the builder type
+    // is TunnelEntry; the ghost then resolves to an entry or exit by hovered position.
+    bool         inTunnelMode() const;
+    // The building type the ghost currently represents: the position-resolved tunnel
+    // type in tunnel mode, otherwise the plain builder type.
+    BuildingType effectiveBuilderType() const;
+    // Recomputes m_tunnelGhostType and m_tunnelPartnerTile from the current ghost
+    // tile, rotation, and sub-tile cursor position. Only meaningful in tunnel mode.
+    void         updateTunnelGhost();
+
     // Belt drag placement (REQ-BLD-BELT-DRAG).
     // Per-path-tile decision, shared by ghost drawing and release-time placement.
     enum class BeltTileAction { PlaceNew, RotateInPlace, Invalid };
@@ -274,6 +284,12 @@ private:
     Rotation                             m_ghostRotation;
     QPoint                               m_ghostTile;
     bool                                 m_ghostValid;
+    // Unified tunnel build mode (REQ-BLD-TUNNEL-MODE): while m_builderType is
+    // TunnelEntry the ghost resolves to an entry or an exit based on the hovered
+    // position; m_tunnelPartnerTile is the existing tunnel it would complete (drawn
+    // as the green connection preview), or unset when there is no completion match.
+    BuildingType                         m_tunnelGhostType = BuildingType::TunnelEntry;
+    std::optional<QPoint>                m_tunnelPartnerTile;
     // Deferred belt drag placement (REQ-BLD-BELT-DRAG): while dragging, the
     // rectilinear anchor->cursor path is recomputed on each move and only applied
     // on release. Empty unless a belt drag is in progress.