Put two people in one fence

The gap the last commit left: a rendered fence is a projection of a shared
text, and nothing asserted that the text was still shared once there was
something drawn from it.

Three facts, and which of them travels is the whole point. Two people typing
at opposite ends of one fence both end up in it, and in each other's
rendering. A language chosen by one is a property of the fence, so it colours
everybody's copy. Hiding the source is a way of looking at a block, so it
hides nobody else's.

The ends of the fence are reached by selecting everything and collapsing the
selection with an arrow. `ControlOrMeta+Home` is not the start of the document
on a Mac, and a keystroke a platform does not recognise is nothing at all —
both edits landed at the end, which reads exactly like a merge that lost one.