mesh/merge-gate pass: builds mesh-host → ace, g14, novox, shanks; no bus step; every machine composes with the change as it did without (4 of 4 compose)
The control-node's walk takes far longer than the engine runs between
updates, so a search that starts over at every restart never completes.
The walk is the engine's own, in lexical order, so its progress is one
cursor per filesystem, kept every minute beside the engine's state and
carried on after a restart while younger than rootsearch.FreshFor. It runs
on a thread of its own at nice 19 and the idle I/O class.
