fix: JS backend compiles print() to console.log()

Bare `print()` calls in Lux now emit `console.log()` in JS output
instead of undefined `print()`. Fixes BUG-006.

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

src/codegen/js_backend.rs

@@ -1066,6 +1066,10 @@ impl JsBackend {
                         let arg = self.emit_expr(&args[0])?;
                         return Ok(format!("String({})", arg));
                     }
+                    if ident.name == "print" {
+                        let arg = self.emit_expr(&args[0])?;
+                        return Ok(format!("console.log({})", arg));
+                    }
                 }
 
                 let arg_strs: Result<Vec<_>, _> = args.iter().map(|a| self.emit_expr(a)).collect();