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/lexer.rs

@@ -70,6 +70,7 @@ pub enum TokenKind {
 
     // Operators
     Plus,       // +
+    PlusPlus,   // ++
     Minus,      // -
     Star,       // *
     Slash,      // /
@@ -160,6 +161,7 @@ impl fmt::Display for TokenKind {
             TokenKind::True => write!(f, "true"),
             TokenKind::False => write!(f, "false"),
             TokenKind::Plus => write!(f, "+"),
+            TokenKind::PlusPlus => write!(f, "++"),
             TokenKind::Minus => write!(f, "-"),
             TokenKind::Star => write!(f, "*"),
             TokenKind::Slash => write!(f, "/"),
@@ -268,7 +270,14 @@ impl<'a> Lexer<'a> {
 
         let kind = match c {
             // Single-character tokens
-            '+' => TokenKind::Plus,
+            '+' => {
+                if self.peek() == Some('+') {
+                    self.advance();
+                    TokenKind::PlusPlus
+                } else {
+                    TokenKind::Plus
+                }
+            }
             '*' => TokenKind::Star,
             '%' => TokenKind::Percent,
             '(' => TokenKind::LParen,