Run behaviors on a fixed step with an On fixed update node

Adding up the frame's dt in On update makes anything that counts in
steps depend on the frame rate. A pitch sweep sounds a little
different on every run. On fixed update runs 60 times per second of
game time with dt exactly 1/60, so the same graph gives the same
result every time.

- fixedSteps in src/core/time.ts is the one rule for the fixed step:
  spend the frame in steps of 1/60, keep the rest, catch up 8 steps
  at most after a hitch.
- The physics now counts its steps with fixedSteps too. It behaves
  as before, and On fixed update runs exactly as often as the engine
  steps.
- The steps run after On mouse wheel and before On update, each as a
  kind of its own with the usual Order setting.
- Each step gets its own context, so a node inside that reads dt
  (Time, Move, Apply force) gets 1/60 rather than the frame.
- A paused game has no steps, also for a behavior that runs while
  paused.
- The engine still takes all of a frame's steps before the behaviors
  run. A force set in On fixed update acts from the next frame on.

Tests:
- fixed-update.test.ts (new): 5 tests. Step counting, uneven frames
  adding up to 60 steps a second, the hitch cap, two steps at 30 Hz
  before On update with dt 1/60 for the event and for Time, one step
  every other frame at 120 Hz, none while paused.
- event-order, node-catalog, physics-system, game-pause: 430 tests
  together with the new file, all green.
- Counter-check: with the frame's dt inside a step and without the
  pause guard both world tests fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/node-catalog.test.ts

@@ -2131,6 +2131,7 @@ const EVENT_NODES: Record<string, string> = {
   OnClick: 'nodes.test.ts',
   OnMouseWheel: 'clip-areas.test.ts',
   OnAnimationFinished: 'animation.test.ts',
+  OnFixedUpdate: 'fixed-update.test.ts',
   OnStateEntered: 'state-machine.test.ts',
   OnStateExited: 'state-machine.test.ts',
 };