Add widget timers to Masonry and use them for delayed UI behavior.

[waywardmonkeys]

• Add understory_timing as a masonry_core dependency.
• Store pending widget timers in RenderRoot.
• Add TimerToken and deliver expired timers through Update::Timer.
• Add context APIs:
  • request_timer(delay) -> TimerToken
  • cancel_timer(token)
• Add backend hooks for timer driving:
  • RenderRoot::set_timer_time
  • RenderRoot::next_timer_deadline
  • RenderRoot::handle_timers
• Update masonry_winit to drive timer time and platform wakeups.
• Convert TextArea cursor blinking to one-shot timers.
• Convert the layers tooltip-delay example to one-shot timers.
• Document when to use timers versus animation frames.

Rationale

Masonry has several UI behaviors that are delayed but not frame-cadenced. Examples include cursor blinking, tooltip
delays, debounce, and long press recognition. These fit a timer model better than animation frames.

This PR adds a widget-local timer lifecycle: widgets request a timer, keep the returned TimerToken, and receive
Update::Timer when it expires. Widgets can cancel pending timers when the relevant state changes.

Animation frames remain the right tool for continuous visual updates, such as interpolation or motion that should advance with frame cadence.

[waywardmonkeys]

This was done with LLM assistance to avoid typing out the boring stuff.

Draft since I will publish understory_timing soon rather than a git dep.