Let a staging clear be scoped to the framework that settled

[SimonCropp]

InlineQueue.Settle is origin scoped, and the reason is on it: a framework that
starts passing removes only its own label, so the other framework's still
failing content stays reviewable. InlineStaging.Clear had no way to say the same
thing, and staging is the same situation - one call site stages one trio per
framework.

So in a net8.0;net10.0 run with no queue owner, where net10 starts passing and
net8 does not, net10's settle deleted net8's trio beside its own. That snapshot
was then pending nowhere at all: not staged, and not in a queue either, because
the reason it was staged is that nothing owned one.

Additive rather than a change of behaviour. Clear takes an optional origin;
passing none clears every framework's trio exactly as before, which is what a
retire wants - the call site is not an inline snapshot in any framework any
more - and is why ClearRemovesEveryFrameworksTrio still passes unchanged. A trio
staged with no framework label is cleared either way, since there is nothing to
scope it by and leaving it would strand it.

Callers that want the queue's semantics can now ask for them; the decision about
which of the two a producer should pass stays with the producer.