#13854 Sector export: Shift BCCON Z-face K indices when pad model is enabled

Pad-model extension assumed OPM-style "Z+" for the positive-Z exterior face,
but ResInsight emits "Z" (OPM-Flow short form). The K1/K2 shift never fired,
leaving the bottom boundary face at the original NZ instead of the new
padded NZ. Match both "Z" and "Z+" so the extension applies.

Adds a regression test verifying that with BCCON_BCPROP and padding the "Z"
exterior face is shifted by nzUpper + nzLower while the "Z-" face remains
at K=1, and documents the exterior-face semantics of the direction mapping.

ApplicationLibCode/ProjectDataModel/Jobs/RimKeywordFactory.cpp

@@ -568,7 +568,12 @@ Opm::DeckKeyword bcconKeyword( const std::vector<RigEclipseResultTools::BorderCe
         return Opm::DeckKeyword();
     }
 
-    // Helper lambda to convert FaceType to Eclipse face string
+    // Helper lambda to convert FaceType to Eclipse BCCON DIRECTION string.
+    // faceType is the direction from the border cell toward its interior neighbour,
+    // so the exterior face of the border cell is on the opposite side. The BCCON
+    // DIRECTION names the exterior face using the OPM-Flow convention: an unsuffixed
+    // axis name is the positive direction, an axis name followed by "-" is the
+    // negative direction.
     auto faceTypeToString = []( cvf::StructGridInterface::FaceType faceType ) -> std::string
     {
         switch ( faceType )