Push/peek/pop including that a peek does not consume and that the flow
under a popped entry survives; that a popped entry stays gone; that a push
past MaxStackDepth returns the evicted entry rather than dropping it
silently; that Put keeps the depth at one; that an expired top takes the
stack with it and is reported once by TakeExpired; and that two dialogue
ids do not read each other's stack.
claude
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