Implement the timer scheduling system, pulled forward from Phase 4
because it is foundational for many guest app patterns.

Callback model:
- Guest calls set_timeout(callback_id, delay_ms) or
  set_interval(callback_id, interval_ms) to schedule timers
- Both return a timer ID for cancellation via clear_timer(id)
- The host fires expired timers by calling the guest-exported
  on_timer(callback_id) function each frame, before on_frame(dt_ms)
- One-shot timers are removed after firing; intervals reschedule
  automatically until cleared
- Timer resolution is tied to the frame rate (~16ms at 60fps)

Host (oxide-browser):
- Remove dead_code annotation from TimerEntry and timers field
- Add timer_next_id counter to HostState for unique timer IDs
- Add drain_expired_timers() utility that collects fired callbacks,
  removes one-shot timers, and reschedules intervals
- Register 3 host functions: api_set_timeout, api_set_interval,
  api_clear_timer
- Extend LiveModule to optionally capture on_timer export and
  fire expired timers in tick() before the on_frame call
- Timer callbacks get their own fuel budget to prevent runaway timers

SDK (oxide-sdk):
- Add set_timeout(), set_interval(), clear_timer() wrappers

Example (examples/timer-demo):
- Countdown timer using set_interval (ticks every 1s from 10 to 0)
- Delayed message using set_timeout (fires after 3 seconds)
- Blinking indicator using set_interval + clear_timer toggle
- Stopwatch using 100ms interval with start/stop/reset controls

Made-with: Cursor