Make pending assignments safe to race, settle them on a tick, and say only what was checked

Review of #150: a withdrawal could land between the look and the act, a
failed ask read as a build in flight, a request kept the wrong asker, a
build being registered read as not built, build "true" could ask a build
nothing waited on, and a status read changed state. Claim a row under the
machine's hold before making it, keep a request only once asked, settle on
the controller's own tick, raise an assignment not made as a condition
until it is answered, and tie each row to its machine.

internal/catalogue/verbs.go

@@ -179,10 +179,11 @@ var ControllerVerbs = []Verb{
 		}, []string{"node"}, "files", "diff")},
 	{Name: "assign", Description: "Put a module on a machine. Refused with the mesh's own words when it cannot resolve there, " +
 		"or when a seat its resources are applied through is held by nothing on the machine (novox/hq ADR 0207). " +
-		"A module not registered yet is looked for (novox/hq issue 325): while its build runs — a merge that adds " +
-		"a module asks for one — the assignment is kept as pending and made when the build registers it, said in " +
-		"`status`, and expires with why if the build fails; a module known and not built is said, and build asks " +
-		"for its build; a name the mesh never heard of is refused with the closest names.",
+		"A module not registered yet is looked for (novox/hq issue 325, ADR 0261): while its build runs — a merge " +
+		"that adds a module asks for one — the assignment is kept as pending and the controller makes it when the " +
+		"build registers it; `status` lists it, and if the build fails it expires with why and raises a condition. " +
+		"A module known and not built is said, and build asks for its build; a name with no module and no build " +
+		"request kept is refused with the closest names.",
 		Input: schema(map[string]string{"node": "the machine's name",
 			"module": "the module's name; several comma-separated are judged together",
 			"build": "\"true\": for a module known and not built (its last build failed, or was never asked), ask " +