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/yield.wren

@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print("fiber 3")
 }
 
-var result = fiber.run // expect: fiber 1
-IO.print("main 1") // expect: main 1
-result = fiber.run // expect: fiber 2
-IO.print("main 2") // expect: main 2
-result = fiber.run // expect: fiber 3
-IO.print("main 3") // expect: main 3
+var result = fiber.call // expect: fiber 1
+IO.print("main 1")  // expect: main 1
+result = fiber.call // expect: fiber 2
+IO.print("main 2")  // expect: main 2
+result = fiber.call // expect: fiber 3
+IO.print("main 3")  // expect: main 3