A decorator that remembers the last output produced by the inner stage and re-emits it when the inner stage yields no value.

The decorator has two internal states:

• **None state** (initial): no output has been seen yet. The inner stage's h8io.stages.Yield.None is forwarded unchanged; a h8io.stages.Yield.Some is forwarded and transitions the wrapper to the Some state.
• **Some state**: a previous output out is remembered. On every subsequent call the inner stage's last known output is emitted regardless of whether the inner stage yields Some or None. A new h8io.stages.Yield.Some updates the remembered value.

In both states the evolution is mapped so that continuations remain wrapped in the appropriate KeepLastOutput variant, preserving the last-value semantics.

The apply factory method always starts in the None state.