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_indirect_reenter.wren

@@ -0,0 +1,20 @@
+var a
+var b
+
+a = new Fiber {
+  IO.print(2)
+  b.run("ignored")
+  IO.print("nope")
+}
+
+b = new Fiber {
+  IO.print(1)
+  a.run("ignored")
+  IO.print(3)
+}
+
+b.call
+// expect: 1
+// expect: 2
+// expect: 3
+IO.print(4) // expect: 4