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_with_value_with_no_caller.wren

@@ -0,0 +1,9 @@
+var a = new Fiber {
+  Fiber.yield(1) // expect runtime error: No fiber to yield to.
+}
+
+// Run a chain of fibers. Since none of them are called, they all get discarded
+// and there is no remaining caller.
+var b = new Fiber { a.run }
+var c = new Fiber { b.run }
+c.run