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/call.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before") // expect: before
+fiber.call         // expect: fiber
+IO.print("after")  // expect: after