fix: support @Module.Const syntax in expressions

[engalar]

Fixes #178

Summary

• Added @QualifiedName as a valid atomicExpression in the MDL grammar for constant references
• New ConstantRefExpr AST node type for clean representation
• Visitor converts @ Module.Const parse tree to ConstantRefExpr
• expressionToString() outputs @Module.Const for BSON serialization
• Backward compatible: getConstantValue('Module.Const') still works

Round-trip restored

-- DESCRIBE outputs:
DECLARE $val String = @MyModule.MyConstant;

-- Can now be fed back into CREATE without modification ✓

Test plan

• mxcli check accepts @Module.Const syntax
• mxcli check still accepts getConstantValue('Module.Const') syntax
• Full test suite passes
• Build succeeds