Core timer

Pure general timer engine. Consumers own state and supply elapsed time; the core does no scheduling or side effects.

let state: State<'exercise'> = empty;
const queued = push([{ kind: 'exercise', duration: 1000 }])(state);
if (queued.ok) state = queued.state;
else showIssues(queued.issues);

const result = tick(400)(state);
if (result.ok) {
  state = result.state;
  // Ordered stages consumed during this committed transition.
  playTransitionEffects(result.effects);
} else {
  showIssues(result.issues); // state is unchanged; effects is empty
}

Stage durations are positive finite numbers no greater than MAX_DURATION = Number.MAX_SAFE_INTEGER. Elapsed input may be zero, but must be finite, nonnegative, and within the same bound. Fractions are supported with ordinary IEEE-754 rounding; exact decimal arithmetic is not promised.

MAX_PROGRAM_STAGES = 10000 bounds eager queue construction. An invalid stage or oversized program rejects the whole operation with issues; the original state is returned and effects is empty.

tick reports every consumed stage in order. pop skips the current stage and restores the next stage; restart restores the current stage; reset empties the queue. Workout durations use integral milliseconds and validate conversion separately in @jikan0/ui.