mesh-issues: an open whose write may have landed never says nothing was opened, and a module must itself be registered

From the review: a taken key that cannot be read back, or a write never
confirmed, now tells the caller to get the number before opening again; an
owner entry naming a module is owned only by that module registered; link
targets get the secret guard; line separators are refused in one-line fields;
the acceptance test now proves the counter's own updates were contended.

modules/mesh-issues/README.md

@@ -48,7 +48,9 @@ bounded number of times with a short random wait; past the bound the open is ref
 - **A refused open takes no number**: an issue is checked before its number is taken.
 - **A number is never handed out twice, and may be skipped.** A number taken and then not kept — the bus
   failing between the counter and the issue — stays unused, and the answer says which. A write whose answer
-  was lost is recognised as its own when tried again, so it leaves no gap.
+  was lost is recognised as its own when tried again, so it leaves no gap; when it cannot be confirmed either
+  way, the answer says the issue may have been written and to `get` that number before opening it again,
+  never that nothing was opened.
 - **An issue is never overwritten.** It is kept by a write that is refused when the key exists; if the
   counter ever hands out a number already kept, the open is refused loudly, the number stays unused, and
   nothing is overwritten.
@@ -85,8 +87,9 @@ verb takes an argument naming a caller, and none can be given one.
 - `first-number` — the number the first issue gets: the one after the highest issue number on hq's main
   and every open branch at the moment the module is switched on. Read once.
 - `owners` — the registered list `located-in` may name: repositories, and modules as
-  `mesh-catalog modules/<module>`. An entry of `located-in` is an owner, or an owner followed by a space and
-  where in it (`mesh-controller internal/broker`). It starts with the mesh's repositories and the modules
+  `mesh-catalog modules/<module>`. An entry of `located-in` is an owner, or an owner followed by a space or a
+  slash and where in it (`mesh-controller internal/broker`, `mesh-catalog modules/nats/module.json`). A module
+  must itself be registered: `mesh-catalog modules/<module>` is never taken as a place in `mesh-catalog`. It starts with the mesh's repositories and the modules
   the catalogue held when this module was written; a module added since is added here through the
   controller's settings verb.