feat: add ++ concat operator and auto-invoke main

BUG-004: Add ++ operator for string and list concatenation across all
backends (interpreter, C, JS) with type checking and formatting support.

BUG-001: Auto-invoke top-level `let main = fn () => ...` when main is
a zero-parameter function, instead of just printing the function value.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/typechecker.rs

@@ -1804,6 +1804,29 @@ impl TypeChecker {
                 }
             }
 
+            BinaryOp::Concat => {
+                // Concat (++) supports strings and lists
+                if let Err(e) = unify_with_env(&left_type, &right_type, &self.env) {
+                    self.errors.push(TypeError {
+                        message: format!("Operands of '++' must have same type: {}", e),
+                        span,
+                    });
+                }
+                match &left_type {
+                    Type::String | Type::List(_) | Type::Var(_) => left_type,
+                    _ => {
+                        self.errors.push(TypeError {
+                            message: format!(
+                                "Operator '++' requires String or List operands, got {}",
+                                left_type
+                            ),
+                            span,
+                        });
+                        Type::Error
+                    }
+                }
+            }
+
             BinaryOp::Sub | BinaryOp::Mul | BinaryOp::Div | BinaryOp::Mod => {
                 // Arithmetic: both operands must be same numeric type
                 if let Err(e) = unify_with_env(&left_type, &right_type, &self.env) {