misc: enhance TreeMapper::map() return type signature

This signature allows auto-completion in PhpStorm when mapping to a
class, while keeping PHPStan and Psalm advanced conditional return
annotation feature.

src/Mapper/TreeMapper.php

@@ -13,7 +13,8 @@ interface TreeMapper
      * @template T of object
      *
      * @param string|class-string<T> $signature
-     * @return (
+     * @return T
+     * @phpstan-return (
      *     $signature is class-string<T>
      *         ? T
      *         : ($signature is class-string ? object : mixed)