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_when_done.wren

@@ -1,6 +1,13 @@
-var fiber = new Fiber {
+var a = new Fiber {
   IO.print("run")
 }
 
-fiber.run // expect: run
-fiber.run // expect runtime error: Cannot run a finished fiber.
+// Run a through an intermediate fiber since it will get discarded and we need
+// to return to the main one after a completes.
+var b = new Fiber {
+  a.run
+  IO.print("nope")
+}
+
+b.call // expect: run
+a.run // expect runtime error: Cannot run a finished fiber.