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/run_with_value.wren

@@ -4,6 +4,8 @@ var fiber = new Fiber {
 
 // The first value passed to the fiber is ignored, since there's no yield call
 // to return it.
-IO.print("before")   // expect: before
+IO.print("before")    // expect: before
 fiber.run("ignored") // expect: fiber
-IO.print("after")    // expect: after
+
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")