Keep a consumer bound where its data is; only a pin moves it (hq ADR 0232, issue 273)

Issue 258's fix let a mesh seat's holder elsewhere answer before this machine's own provider. Right
for the resolver, which any provider answers alike; for the store's seat it re-bound every database
consumer on a machine running its own store to the holder on another, each was given a fresh, empty
database there, and nothing said so for twenty hours.

- An offer says whether it keeps its consumers' data (`keeps-consumer-data`); unsaid, a provider
  that grants each consumer a credential does. For such a provision the seat's holder no longer
  overrules a provider beside the consumer; a pin still does.
- Where each such consumer was sent is recorded (migration 0071). A resolution that would bind it
  elsewhere keeps the recorded provider and says the move; one whose provider is gone is refused,
  never answered by another.
- A push says a kept move and raises it as an urgent condition at once; the self-check's D12 raises
  it every run, with a pinned move not yet sent as a warning and any unasked move as urgent.

cmd/mesh-controller/doctor.go

@@ -99,6 +99,9 @@ var probeRegistry = []probe{
 		"a plan's window", From: "the version split", Kind: "core-behind", Phase: 1, run: probeCoreBuilds},
 	{ID: "D11", Asserts: "no provider holds a consumer retired more than thirty days without a person deciding " +
 		"its cleanup", From: "ADR 0230", Kind: kindCleanupWaiting, Phase: 2, run: probeRetired},
+	{ID: probeBindingsID, Asserts: "every consumer of a provision that keeps its data is bound where it was last " +
+		"sent, or moves by a pin", From: "issue 273, ADR 0232", Kind: kindBindingMoved,
+		Raises: []string{kindBindingKept, kindBindingMoving}, Phase: 2, run: probeBindings},
 	{ID: "DW", Asserts: "the watchdogs of the signals table ran within three of their intervals",
 		From: "ADR 0227 rule 6: the watchers are watched", Kind: "watchdogs-silent", Phase: 1, run: probeWatchdogs},
 }