Prevent backannotation from changing variables

Updating to PCB always resolves text variables from the schematic.  This
means that back annotation will always start from resolved variables if
they exist in the schematic.  So to avoid this, we prevent overwriting
any text variables that exist in the schematic.

This does prevent the user from updating a text variable in a pcbnew
field and backannotating this variable change into the schematic where
the schematic had a text variable already.  Text variables are forward
annotation only until we add an additional property to each field to
include the unresolved text

Fixes https://gitlab.com/kicad/code/kicad/issues/21724

eeschema/tools/backannotate.cpp

@@ -384,7 +384,8 @@ void BACK_ANNOTATE::applyChangelist()
         if( !m_dryRun )
             commit.Modify( symbol, screen, RECURSE_MODE::NO_RECURSE );
 
-        if( m_processReferences && ref.GetRef() != fpData.m_ref && !skip )
+        if( m_processReferences && ref.GetRef() != fpData.m_ref && !skip
+                && !symbol->GetField( FIELD_T::REFERENCE )->HasTextVars() )
         {
             ++m_changesCount;
             msg.Printf( _( "Change %s reference designator to '%s'." ),
@@ -397,7 +398,8 @@ void BACK_ANNOTATE::applyChangelist()
             m_reporter.ReportHead( msg, RPT_SEVERITY_ACTION );
         }
 
-        if( m_processFootprints && oldFootprint != fpData.m_footprint && !skip )
+        if( m_processFootprints && oldFootprint != fpData.m_footprint && !skip
+                && !symbol->GetField( FIELD_T::FOOTPRINT )->HasTextVars() )
         {
             ++m_changesCount;
             msg.Printf( _( "Change %s footprint assignment from '%s' to '%s'." ),
@@ -411,7 +413,8 @@ void BACK_ANNOTATE::applyChangelist()
             m_reporter.ReportHead( msg, RPT_SEVERITY_ACTION );
         }
 
-        if( m_processValues && oldValue != fpData.m_value && !skip )
+        if( m_processValues && oldValue != fpData.m_value && !skip
+                && !symbol->GetField( FIELD_T::VALUE )->HasTextVars() )
         {
             ++m_changesCount;
             msg.Printf( _( "Change %s value from '%s' to '%s'." ),
@@ -502,6 +505,7 @@ void BACK_ANNOTATE::applyChangelist()
                 // with all the variables resolved. The footprints field value gets the symbol's
                 // resolved value when the PCB is updated from the schematic.
                 if( symField
+                    && !symField->HasTextVars()
                     && symField->GetShownText( &ref.GetSheetPath(), false ) != fpFieldValue )
                 {
                     m_changesCount++;