Fix an incorrect calculation in length tuning tools.

MEANDER_PLACER_BASE::lineLength wasn't calculating the length correctly
when starting and ending pads were on different layers.

Naturally, now jointA corresponds to the start of the path, jointB to the end.

Fixes https://gitlab.com/kicad/code/kicad/issues/12881

pcbnew/router/pns_topology.cpp

@@ -304,8 +304,8 @@ const ITEM_SET TOPOLOGY::AssembleTrivialPath( ITEM* aStart,
     JOINT* jointA = nullptr;
     JOINT* jointB = nullptr;
 
-    followTrivialPath( &l, false, path, visited, &jointA );
-    followTrivialPath( &l, true, path, visited, &jointB );
+    followTrivialPath( &l, false, path, visited, &jointB );
+    followTrivialPath( &l, true, path, visited, &jointA );
 
     if( aTerminalJoints )
     {