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/closure.wren

@@ -17,9 +17,9 @@ var closure
   }
 }
 
-fiber.run    // expect: before
+fiber.call   // expect: before
 closure.call // expect: before
-fiber.run
+fiber.call
 closure.call // expect: after
-fiber.run    // expect: after
+fiber.call   // expect: after
 closure.call // expect: final