ConsumersOf's carried-module path (a module the tool runner carries, no
account of its own — mesh-mail's real shape) was not passing Queues
through, so its work-queue consumer was missing from the composed list
and PruneHeard would have removed what issueProcess made for it. Fixed,
with a test through Users -> ConsumersOf.
Extracted EnsureStream's config building into StreamConfigFor, a pure
function, so the dead-letters test can assert the restricted metadata
EnsureStream would actually send for a work-queue stream instead of
setting the dead letter's header by hand, which passed whether or not
modstreams.go's out.Restricted = true was there at all.
jochen