A pin names the module as well as the node; a node that answers twice is refused

A provider is a (node, module) pair (design 23), and the pin — the one way a
consumer names its provider — named only the node. Two modules on one node
can both answer a provision (public-acme and step-ca both offer acme-ca on
novox), and then the resolver, given a pin naming that node, took the last
provider listed: a coin flip. The same ambiguity beside the consumer was
settled by a map walk — random per plan — which is how novox's own
route-proxy got its issuer (novox/hq #258).

- `pin <node> <provision> <from-node> <module>`: both halves, always. The
  console gains `pin` and `unpin`. The provider may be on the consumer's own
  node, since two modules beside it can both answer.
- The resolver refuses ambiguity instead of picking, across machines and
  beside the consumer alike, naming every candidate as node/module and the
  form of the pin that settles it. A plain capability that grants nothing
  and serves nothing (three shells beside an editor) is not a choice to put
  to anybody and stays as it was.
- provision_pin gains a nullable module (0050); records made before are
  completed where the node they name answers once, and left for a person
  where it answers twice (0051).
- The provider of something already satisfied is looked for among what was
  assigned, not only what the walk has reached — a consumer reached before
  the provider beside it no longer loses its binding.
- The start-time check that every declared verb is runnable samples each
  verb's required arguments from its schema instead of three guessed keys.

Live consequence: a node that has two providers of one bound provision
assigned (novox: acme-ca) resolves only once pinned —
`pin novox acme-ca novox public-acme`.

internal/inventory/migrations/0050-a-pin-names-the-module.sql

@@ -0,0 +1,12 @@
+-- A pin names the module as well as the node (novox/hq #258).
+--
+-- 0008 said "not a module: the same module on two machines is two answers, and which machine is the
+-- whole question". Half right. Two modules on one machine can both answer a provision — public-acme
+-- and step-ca both offer acme-ca on novox — and then which *module* is the whole question, and a
+-- node alone cannot ask it. The resolver, given a node that answered twice, took the last one listed.
+--
+-- A provider is a (node, module) pair (design 23), and a pin names the pair. Nullable, so a record
+-- made before this was asked keeps meaning what it meant: honoured while that node answers once,
+-- refused with the module asked for when it answers twice.
+alter table provision_pin add column module text;
+