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_with_value.wren

@@ -0,0 +1,9 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")    // expect: before
+fiber.call("ignored") // expect: fiber
+IO.print("after")     // expect: after