Eeschema: Add multi-unit symbol support for text variables

Add enhancements to make working with multi-unit symbols easier,
particularly for connector pinout tables.

New features:

1. UNIT(<pin_number>) function
   Returns the full unit reference for a given pin number.
   Example: ${J601:UNIT(1)} returns "J601A" if pin 1 is in unit A.

2. Parent reference support in cross-references
   Enables using parent reference (J601) instead of full unit reference
   (J601A) in text variable expressions.
   Example: ${J601:PIN_NAME(1)} now works (previously required ${J601A:PIN_NAME(1)})

3. Auto-resolution across units
   PIN_NAME, NET_NAME, NET_CLASS, and SHORT_NET_NAME functions now
   automatically search all units if a pin is not found in the current unit.

4. NC display for unconnected pins
   SHORT_NET_NAME now displays "NC" instead of "unconnected-(Ref-Pin_X)",
   making tables cleaner. NET_NAME still shows the full technical name.

Implementation:
- Enhanced SCHEMATIC::ResolveCrossReference() to add reference-based
  lookup as a fallback after UUID resolution
- Added UNIT() function to SCH_SYMBOL::ResolveTextVar()
- Modified SHORT_NET_NAME handling to detect and format unconnected nets

eeschema/sch_symbol.cpp

@@ -1535,6 +1535,7 @@ void SCH_SYMBOL::GetContextualTextVars( wxArrayString* aVars ) const
     aVars->push_back( wxT( "NET_NAME(<pin_number>)" ) );
     aVars->push_back( wxT( "NET_CLASS(<pin_number>)" ) );
     aVars->push_back( wxT( "PIN_NAME(<pin_number>)" ) );
+    aVars->push_back( wxT( "UNIT(<pin_number>)" ) );
 }
 
 
@@ -1774,16 +1775,33 @@ bool SCH_SYMBOL::ResolveTextVar( const SCH_SHEET_PATH* aPath, wxString* token, i
     else if( token->StartsWith( wxT( "SHORT_NET_NAME(" ) )
                  || token->StartsWith( wxT( "NET_NAME(" ) )
                  || token->StartsWith( wxT( "NET_CLASS(" ) )
-                 || token->StartsWith( wxT( "PIN_NAME(" ) ) )
+                 || token->StartsWith( wxT( "PIN_NAME(" ) )
+                 || token->StartsWith( wxT( "UNIT(" ) ) )
     {
         wxString pinNumber = token->AfterFirst( '(' );
         pinNumber = pinNumber.BeforeLast( ')' );
 
-        for( SCH_PIN* pin : GetPins( aPath ) )
+        // First, try to find the pin in the current unit (for backward compatibility)
+        // For UNIT(), always search all pins to find which unit the pin belongs to
+        std::vector<SCH_PIN*> pinsToSearch = token->StartsWith( wxT( "UNIT(" ) )
+                                              ? GetAllLibPins()
+                                              : GetPins( aPath );
+
+        for( SCH_PIN* pin : pinsToSearch )
         {
             if( pin->GetNumber() == pinNumber )
             {
-                if( token->StartsWith( wxT( "PIN_NAME" ) ) )
+                if( token->StartsWith( wxT( "UNIT(" ) ) )
+                {
+                    // Return the full unit reference (e.g., "J601A")
+                    int pinUnit = pin->GetUnit();
+                    if( pinUnit > 0 )
+                        *token = GetRef( aPath, false ) + SubReference( pinUnit, false );
+                    else
+                        *token = GetRef( aPath, false );
+                    return true;
+                }
+                else if( token->StartsWith( wxT( "PIN_NAME" ) ) )
                 {
                     *token = pin->GetAlt().IsEmpty() ? pin->GetName() : pin->GetAlt();
                     return true;
@@ -1794,7 +1812,13 @@ bool SCH_SYMBOL::ResolveTextVar( const SCH_SHEET_PATH* aPath, wxString* token, i
                 if( !conn )
                     *token = wxEmptyString;
                 else if( token->StartsWith( wxT( "SHORT_NET_NAME" ) ) )
-                    *token = conn->LocalName();
+                {
+                    wxString netName = conn->LocalName();
+                    if( netName.Lower().StartsWith( wxT( "unconnected" ) ) )
+                        *token = wxT( "NC" );
+                    else
+                        *token = netName;
+                }
                 else if( token->StartsWith( wxT( "NET_NAME" ) ) )
                     *token = conn->Name();
                 else if( token->StartsWith( wxT( "NET_CLASS" ) ) )
@@ -1803,6 +1827,45 @@ bool SCH_SYMBOL::ResolveTextVar( const SCH_SHEET_PATH* aPath, wxString* token, i
                 return true;
             }
         }
+
+        // If pin not found in current unit, search all units (auto-resolution)
+        if( !token->StartsWith( wxT( "UNIT(" ) ) )
+        {
+            for( SCH_PIN* pin : GetAllLibPins() )
+            {
+                if( pin->GetNumber() == pinNumber )
+                {
+                    if( token->StartsWith( wxT( "PIN_NAME" ) ) )
+                    {
+                        *token = pin->GetAlt().IsEmpty() ? pin->GetName() : pin->GetAlt();
+                        return true;
+                    }
+
+                    // For net-related functions, we need to find the correct sheet path
+                    // that has this pin's unit selected
+                    // Note: This is a simplification - in practice we may need to search
+                    // for the actual instance with this unit
+                    SCH_CONNECTION* conn = pin->Connection( aPath );
+
+                    if( !conn )
+                        *token = wxEmptyString;
+                    else if( token->StartsWith( wxT( "SHORT_NET_NAME" ) ) )
+                    {
+                        wxString netName = conn->LocalName();
+                        if( netName.Lower().StartsWith( wxT( "unconnected" ) ) )
+                            *token = wxT( "NC" );
+                        else
+                            *token = netName;
+                    }
+                    else if( token->StartsWith( wxT( "NET_NAME" ) ) )
+                        *token = conn->Name();
+                    else if( token->StartsWith( wxT( "NET_CLASS" ) ) )
+                        *token = pin->GetEffectiveNetClass( aPath )->GetName();
+
+                    return true;
+                }
+            }
+        }
     }
 
     // See if parent can resolve it (this will recurse to ancestors)

eeschema/schematic.cpp

@@ -749,6 +749,53 @@ bool SCHEMATIC::ResolveCrossReference( wxString* token, int aDepth ) const
         return true;    // Cross-reference is resolved whether or not the actual textvar was
     }
 
+    // If UUID resolution failed, try to resolve by reference designator
+    // This handles both exact matches (J601A) and parent references for multi-unit symbols (J601)
+    if( !refItem )
+    {
+        SCH_REFERENCE_LIST refs;
+        Hierarchy().GetSymbols( refs );
+
+        SCH_SYMBOL*    foundSymbol = nullptr;
+        SCH_SHEET_PATH foundPath;
+
+        for( int ii = 0; ii < (int) refs.GetCount(); ii++ )
+        {
+            SCH_REFERENCE& reference = refs[ii];
+            wxString       symbolRef = reference.GetSymbol()->GetRef( &reference.GetSheetPath(), false );
+
+            // Try exact match first
+            if( symbolRef == ref )
+            {
+                foundSymbol = reference.GetSymbol();
+                foundPath = reference.GetSheetPath();
+                break;
+            }
+
+            // For multi-unit symbols, try matching parent reference (e.g., J601 matches J601A)
+            if( symbolRef.StartsWith( ref ) && symbolRef.Length() == ref.Length() + 1 )
+            {
+                wxChar lastChar = symbolRef.Last();
+                if( lastChar >= 'A' && lastChar <= 'Z' )
+                {
+                    foundSymbol = reference.GetSymbol();
+                    foundPath = reference.GetSheetPath();
+                    // Don't break - continue looking for exact match
+                }
+            }
+        }
+
+        if( foundSymbol )
+        {
+            if( foundSymbol->ResolveTextVar( &foundPath, &remainder, aDepth + 1 ) )
+                *token = std::move( remainder );
+            else
+                *token = foundSymbol->GetRef( &foundPath, true ) + wxS( ":" ) + remainder;
+
+            return true;
+        }
+    }
+
     return false;
 }