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/resume_caller.wren

@@ -4,12 +4,12 @@ var b = new Fiber {
 
 var a = new Fiber {
   IO.print("begin fiber a")
-  b.run
+  b.call
   IO.print("end fiber a")
 }
 
 IO.print("begin main")
-a.run
+a.call
 IO.print("end main")
 
 // expect: begin main