Add `String.safeToString_` static method that tracks objects currently being
converted to string per fiber, returning "..." when recursion is detected.
Wrap List.toString and Map.toString with this guard to handle self-referential
and mutually recursive structures without crashing.
The benchmark directory is relocated from the project root into the test directory, and all scripts, documentation links, and gitignore paths are updated accordingly. The test runner is also refactored to explicitly walk subdirectories (core, io, language, limit) instead of using a single test root, and a new test/README.md is added to document the test suite structure. Additionally, the constructor test for the Foo class is updated to add a zero-arity constructor and adjust expected output strings.
