Time events: transitions that fire after a delay in a state #703

Why

Timers are among the most requested features in every state machine library surveyed: pytransitions/transitions#198 became its Timeout feature, statelyai/xstate#36, many spring-statemachine issues, and here #434 and discussion #549. delay= already exists on send and contrib.timeout provides a timeout invoke. Previously out of scope for #692 because PSSM has no time-event tests; reopened by demand.

Scope

• A declarative spelling for "after N seconds in this state" (after= is already taken by callbacks), built on the timeout invoke so it is cancelled when the state exits.
• Not built on delayed sends: a sync send(delay=) from a callback blocks the processing thread (Sync engine: send(delay=) from a callback blocks the processing thread #706).
• YAML/schema and diagram label.

Acceptance

Tests on both engines covering firing, cancellation on exit, and re-entry restarting the timer.

Refs #692