Highlight tunnel connections in green when selected

src/lib/core/TunnelCompletion.h

@@ -52,3 +52,15 @@ struct TunnelCompletion
 TunnelCompletion resolveTunnelCompletion(const TunnelLookup& lookup, QPoint hoverTile,
                                          Rotation rotation, int maxDistance,
                                          QVector2D cursorWorldPos);
+
+// Finds the matching end of an existing tunnel building at `tile` of the given `type`
+// (TunnelEntry or TunnelExit) facing `rotation`, applying the pairing scan of
+// REQ-BLD-TUNNEL-PAIR over `lookup`:
+//  - for an entry, the first same-direction tunnel along its facing direction, if it
+//    is an exit;
+//  - for an exit, the first same-direction tunnel opposite its facing direction, if it
+//    is an entry.
+// Returns the partner tile, or std::nullopt when the tunnel is unpaired.
+std::optional<QPoint> findTunnelPartner(const TunnelLookup& lookup, QPoint tile,
+                                        BuildingType type, Rotation rotation,
+                                        int maxDistance);