feat: replace fiber run() with transfer() and suspend() for scheduler-friendly semantics

Remove the run() method from fibers and introduce transfer() for non-stack-based fiber switching, along with suspend() to pause the interpreter and return control to the host application. Update Timer.sleep() to use runNextScheduled_() instead of Fiber.yield(), and add Timer.schedule() for creating independently scheduled fibers. Refactor the core fiber runtime to support transfer semantics, including proper error handling for aborted fibers and self-transfer edge cases. Fix a missing quote in test.py's runtime error validation string.

script/test.py

@@ -149,7 +149,7 @@ class Test:
 
   def validate_runtime_error(self, error_lines):
     if len(error_lines) < 2:
-      self.fail('Expected runtime error "{0} and got none.',
+      self.fail('Expected runtime error "{0}" and got none.',
           self.runtime_error_message)
       return