feat: implement symmetric coroutines with fiber.call and fiber.run distinction

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.

test/fiber/is_done.wren

@@ -5,7 +5,7 @@ var fiber = new Fiber {
 }
 
 IO.print(fiber.isDone)  // expect: false
-fiber.run               // expect: 1
+fiber.call              // expect: 1
 IO.print(fiber.isDone)  // expect: false
-fiber.run               // expect: 2
+fiber.call              // expect: 2
 IO.print(fiber.isDone)  // expect: true