Class Ladder

Creates a ladder with no rungs, buffering into a store.

The store is consumed: the ladder owns it from here on.

How many messages are waiting in the buffer.

Connects every rung, so a send can be tried against each in turn.

A rung that will not connect is left in the ladder: it may come back, and a send simply falls through it until it does.

Replays the buffer over the rungs, oldest message first, and reports how many went out.

Adds a rung, which is tried after the rungs already added.

Add the cheapest, most-preferred link first and the costliest fallback last, because a send takes the first rung that accepts it. The transport is consumed.

Sends a payload, falling through the rungs and buffering if none take it.

Buffering is a success, not a failure: it is what the ladder exists to do.