- Change Fiber.yield() to exit interpreter when no parent fiber exists instead of raising runtime error
- Add Fiber.current primitive to return the currently executing fiber object
- Update interpreter loop to stop execution when PRIM_RUN_FIBER receives null value
- Modify test expectations to reflect new yielding behavior from main fiber
- 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 new `fiber.call` method that remembers the calling fiber for symmetric transfer, while `fiber.run` now performs tail-call style switching without preserving the caller. Introduce re-entrancy guards preventing a fiber from being called multiple times, update all test fixtures to use the new API, and add comprehensive test coverage for call semantics including direct/indirect re-entrancy, value passing, and finished fiber errors.
