fix: support @Module.Const syntax in expressions (fixes mendixlabs#178)

Parser now accepts @QualifiedName as a constant reference in any
expression context (DECLARE, SET, CHANGE, etc.), matching the output
of DESCRIBE MICROFLOW. This restores the round-trip guarantee:
DESCRIBE output can be fed back into CREATE without modification.

Author: engalar

mdl/ast/ast_expression.go

@@ -108,6 +108,13 @@ type QualifiedNameExpr struct {
 
 func (e *QualifiedNameExpr) isExpression() {}
 
+// ConstantRefExpr represents a constant reference: @Module.ConstantName
+type ConstantRefExpr struct {
+	QualifiedName QualifiedName // The constant qualified name
+}
+
+func (e *ConstantRefExpr) isExpression() {}
+
 // IfThenElseExpr represents an inline if-then-else expression:
 // if condition then trueExpr else falseExpr
 type IfThenElseExpr struct {

mdl/executor/cmd_microflows_helpers.go

@@ -130,6 +130,8 @@ func expressionToString(expr ast.Expression) string {
 	case *ast.QualifiedNameExpr:
 		// Qualified name (association name, entity reference) - unquoted
 		return e.QualifiedName.String()
+	case *ast.ConstantRefExpr:
+		return "@" + e.QualifiedName.String()
 	case *ast.IfThenElseExpr:
 		cond := expressionToString(e.Condition)
 		thenStr := expressionToString(e.ThenExpr)

mdl/grammar/MDLParser.g4

@@ -3495,6 +3495,7 @@ argumentList
 atomicExpression
     : literal
     | VARIABLE (DOT attributeName)*    // $Var or $Widget.Attribute (data source ref)
+    | AT qualifiedName                 // @Module.ConstantName (constant reference)
     | qualifiedName
     | IDENTIFIER
     | MENDIX_TOKEN

mdl/grammar/parser/MDLParser.interp

@@ -617,6 +617,15 @@ func buildAtomicExpression(ctx parser.IAtomicExpressionContext) ast.Expression {
 		}
 	}
 
+	// Constant reference: @Module.ConstantName
+	if atomCtx.AT() != nil {
+		if qn := atomCtx.QualifiedName(); qn != nil {
+			return &ast.ConstantRefExpr{
+				QualifiedName: buildQualifiedName(qn),
+			}
+		}
+	}
+
 	// Mendix token [%TokenName%]
 	if token := atomCtx.MENDIX_TOKEN(); token != nil {
 		tokenText := token.GetText()