Reset runtime and preempted in rb_fiber_atfork

The child process inherits the parent's root fiber state at fork time.
rb_fiber_atfork already reset quantum to the default but left runtime
and preempted untouched. A child forked while a fiber had accumulated
runtime or had preempted=1 would start with stale scheduling state,
potentially triggering an immediate spurious preemption on the first
back-edge check. Reset all three fields consistently, matching what
rb_threadptr_root_fiber_setup does for a freshly created root fiber.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: samuel-williams-shopify

cont.c

@@ -3591,7 +3591,9 @@ rb_fiber_atfork(rb_thread_t *th)
         }
         th->root_fiber->prev = 0;
         th->root_fiber->blocking = 1;
-        th->root_fiber->cont.saved_ec.quantum = RUBY_FIBER_QUANTUM_DEFAULT;
+        th->root_fiber->cont.saved_ec.runtime   = 0;
+        th->root_fiber->cont.saved_ec.preempted = 0;
+        th->root_fiber->cont.saved_ec.quantum   = RUBY_FIBER_QUANTUM_DEFAULT;
         th->blocking = 1;
     }
 }