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/ast.rs

@@ -621,7 +621,8 @@ pub enum BinaryOp {
     And,
     Or,
     // Other
-    Pipe, // |>
+    Pipe,   // |>
+    Concat, // ++
 }
 
 impl fmt::Display for BinaryOp {
@@ -641,6 +642,7 @@ impl fmt::Display for BinaryOp {
             BinaryOp::And => write!(f, "&&"),
             BinaryOp::Or => write!(f, "||"),
             BinaryOp::Pipe => write!(f, "|>"),
+            BinaryOp::Concat => write!(f, "++"),
         }
     }
 }