- Compile empty `()` as a valid method signature distinct from no parentheses
- Update `call()`, `clear()`, `run()`, `try()`, and `yield()` to use empty argument lists
- Revise documentation across multiple files to reflect new signature semantics
Add a new `isDone` native method to the Fiber class that returns true when the fiber has no remaining frames. Replace TODO comments with proper runtime error handling in `fiber_run`, `fiber_run1`, `fiber_yield`, and `fiber_yield1` to prevent running finished fibers or yielding from the main fiber. Include test cases verifying the new property and error conditions.