feat: auto-invoke let main in JS backend

The JS backend now detects `let main = fn() => ...` patterns and
auto-invokes them at the end of the generated code, matching the
interpreter's behavior.

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

src/codegen/js_backend.rs

@@ -187,6 +187,20 @@ impl JsBackend {
             }
         }
 
+        // Check for `let main = fn() => ...` pattern (not tracked in self.functions)
+        let has_let_main = program.declarations.iter().any(|d| {
+            if let Declaration::Let(l) = d {
+                l.name.name == "main" && matches!(&l.value, Expr::Lambda { .. })
+            } else {
+                false
+            }
+        });
+        if has_let_main && !self.functions.contains("main") && !has_main_call {
+            self.writeln("");
+            self.writeln("// Entry point (let main)");
+            self.writeln("main();");
+        }
+
         Ok(self.output.clone())
     }
 

src/main.rs

@@ -4286,6 +4286,31 @@ c")"#;
         assert!(js.contains("Lux.None"), "JS should contain Lux.None for List.get: {}", js);
     }
 
+    #[test]
+    fn test_let_main_js_codegen() {
+        use crate::codegen::js_backend::JsBackend;
+        use crate::parser::Parser;
+        use crate::lexer::Lexer;
+
+        let source = r#"
+            let main = fn() => {
+                print("hello from let main")
+            }
+        "#;
+
+        let tokens = Lexer::new(source).tokenize().unwrap();
+        let program = Parser::new(tokens).parse_program().unwrap();
+        let mut backend = JsBackend::new();
+        let js = backend.generate(&program).unwrap();
+
+        // Should contain the let binding
+        assert!(js.contains("const main"), "JS should contain 'const main': {}", js);
+        // Should auto-invoke main()
+        assert!(js.contains("main();"), "JS should auto-invoke main(): {}", js);
+        // Should NOT contain main_lux (let bindings aren't mangled)
+        assert!(!js.contains("main_lux"), "let main should not be mangled: {}", js);
+    }
+
     #[test]
     fn test_invalid_escape_sequence() {
         let result = eval(r#"let x = "\z""#);