Add slide metadata for timer actions on advance

[samuel-williams-shopify]

A waiting/title slide currently requires a separate click on Start when the talk begins. Add timer frontmatter so advancing away from that slide can start timing automatically:

template: title
duration: 0
timer: start

The field uses the existing timer vocabulary: start starts an unstarted timer, pause freezes elapsed time, and resume continues a started timer. Actions apply to the outgoing slide on a successful Next action in the presenter or display. Revisiting a start slide preserves elapsed time and manual pauses.

Backward navigation, jumps, reloads, reconnects, saved-state restoration, and recorder navigation do not trigger timer actions. The new timer state is persisted and sent to listeners together with the new slide position. Zero-duration title/break slides are documented, and progress calculation handles them without dividing by zero: progress is 0.0 before the timer starts or before the slide's expected start, then 1.0 at and after its expected start.

Slide#duration returns a finite, non-negative Float, accepting numeric strings and using 0.0 for negative, malformed, or non-finite values. Missing or null metadata also defaults to 0.0, meaning no time has been allocated. Presentations that relied on the previous implicit 60-second duration need explicit durations to retain that pacing schedule. This keeps total duration and pacing consistent and preserves fractional progress when elapsed time is an integer.

When a presentation has no allocated time, the presenter keeps elapsed time and timer controls visible while hiding pacing indicators, slide progress, and the remaining-time estimate.

The recorder uses advance!(timer: false) to keep navigation in the controller while opting out of live timer actions. The default advance! applies timer actions. Both delegate to go_to, which validates the destination before changing the timer or slide and returns true for a valid destination or false otherwise.

Validation: bundle exec bake test passes (276 tests, 598 assertions); bundle exec rubocop passes (81 files). Coverage includes navigation return values and invalid destinations, presenter/display navigation, recorder isolation, start/pause/resume behavior and opting out, persistence, duration normalization and defaults, untimed presentations, and progress for zero or negative durations before, at, and after the expected start.

[samuel-williams-shopify]

@sampokuokkanen wdyt of this?