Exact clearance is allowed; less than clearance is an error.

Also fixes a bug where the units weren't shown for extra precision
numbers.

Fixes https://gitlab.com/kicad/code/kicad/issues/13262

pcbnew/drc/drc_test_provider.cpp

@@ -371,8 +371,8 @@ wxString DRC_TEST_PROVIDER::formatMsg( const wxString& aFormatString, const wxSt
     if( constraint_str == actual_str )
     {
         // Use more precise formatting if the message-text strings were equal.
-        constraint_str = StringFromValue( aConstraint );
-        actual_str = StringFromValue( aActual );
+        constraint_str = StringFromValue( aConstraint, true );
+        actual_str = StringFromValue( aActual, true );
     }
 
     return wxString::Format( aFormatString, aSource, constraint_str, actual_str );

pcbnew/drc/drc_test_provider_edge_clearance.cpp

@@ -102,6 +102,10 @@ bool DRC_TEST_PROVIDER_EDGE_CLEARANCE::testAgainstEdge( BOARD_ITEM* item, SHAPE*
     {
         if( itemShape->Collide( shape.get(), minClearance, &actual, &pos ) )
         {
+            // Exact clearance is allowed
+            if( minClearance > 0 && actual == minClearance )
+                return true;
+
             if( item->Type() == PCB_TRACE_T || item->Type() == PCB_ARC_T )
             {
                 // Edge collisions are allowed inside the holes of castellated pads