Reset runtime and preempted in rb_fiber_atfork, add test

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 a spurious immediate 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.

Add test_fork_resets_fiber_scheduling_state to verify that a child
forked from a fiber with accumulated runtime starts with runtime near
zero and quantum at the default value.

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

Author: samuel-williams-shopify

cont.c

@@ -2998,7 +2998,6 @@ rb_fiber_runtime(VALUE self)
     return UINT2NUM(fiber->cont.saved_ec.runtime);
 }
 
-
 /*
  * call-seq: fiber.quantum -> integer
  *

test/ruby/test_fiber.rb

@@ -427,6 +427,39 @@ def test_fork_from_fiber
     assert_predicate(status, :success?)
   end
 
+  def test_fork_resets_fiber_scheduling_state
+    omit 'fork not supported' unless Process.respond_to?(:fork)
+    omit "Fiber#runtime counter requires interpreter dispatch" if
+      defined?(RubyVM::YJIT) && RubyVM::YJIT.enabled?
+
+    # Accumulate runtime and set preempted in the parent so that the child
+    # inherits a dirty scheduling state, then verify rb_fiber_atfork resets it.
+    r, w = IO.pipe
+
+    f = Fiber.new(blocking: false) do
+      10_000.times { }  # accumulate runtime in parent
+      pid = fork do
+        r.close
+        fiber = Fiber.current
+        w.write("#{fiber.runtime},#{fiber.quantum}")
+        w.close
+        exit!(0)
+      end
+      w.close
+      child_output = r.read
+      Process.waitpid(pid)
+      child_output
+    end
+    result = f.transfer
+    r.close
+
+    child_runtime, child_quantum = result.split(",").map(&:to_i)
+    assert_operator child_runtime, :<, 100,
+      "child runtime=#{child_runtime} was not reset by rb_fiber_atfork (expected < 100)"
+    assert_equal 50_000, child_quantum,
+      "child quantum was not reset to default (got #{child_quantum})"
+  end
+
   def test_exit_in_fiber
     bug5993 = '[ruby-dev:45218]'
     assert_nothing_raised(bug5993) do