Keep a named push from sending builds a policy or a plan holds back

A named push flushed every other machine whose declaration differed from
what it was last sent (hq ADR 0083). Under an upgrade policy of `record`,
or a plan still waiting on its first machine (ADR 0218), every machine
running the module differs, so `push <one>` sent the held build to all of
them (hq issue 259).

Each send now records which build of each module it carried
(node.sent_builds, migration 0061). The cascade, and the bus holder added
to a named push, skip a machine any of whose modules would move to a
build its policy records or an open plan has not sent it, and say which
module, which build, why, and that `push <node>` sends it. A machine
whose last send was not recorded is held until it is named. The named
machine itself, a whole-mesh push and `push --behind` are unchanged.

cmd/mesh-controller/plan.go

@@ -397,9 +397,49 @@ func declarationWith(ctx context.Context, open *stores, node string,
 	if err != nil {
 		return sendable{}, err
 	}
-	return sendable{Resources: composed.Resources, Adoption: adoption,
+	out := sendable{Resources: composed.Resources, Adoption: adoption,
 		Received: composed.Received, Mesh: with.Mesh, BusUsers: with.BusUsers,
-		LeftOut: sortedKeysOf(composed.LeftOut), leftOutWhy: composed.LeftOut}, nil
+		LeftOut: sortedKeysOf(composed.LeftOut), leftOutWhy: composed.LeftOut}
+	// And which build of each module it carries, for the send to record (novox/hq issue 259, ADR
+	// 0221). Read only on the send path: a question about what would be sent records nothing.
+	if choosing == Allocating {
+		current, err := open.inventory.CurrentBuilds(ctx)
+		if err != nil {
+			return sendable{}, err
+		}
+		before, known, err := open.inventory.SentBuilds(ctx, node)
+		if err != nil {
+			return sendable{}, err
+		}
+		if !known {
+			before = nil
+		}
+		names := make([]string, 0, len(plan.Modules))
+		for _, m := range plan.Modules {
+			names = append(names, m.Module)
+		}
+		out.Builds = carriedBuilds(names, composed.LeftOut, current, before)
+	}
+	return out, nil
+}
+
+// carriedBuilds is the build of each module a declaration carries, as a send records it (novox/hq
+// issue 259): the module's current build for each module in it, and for a module left out of it
+// (ADR 0163, rule 6) the build it was last sent, since the machine keeps that one — or nothing, when
+// that is not known. Never nil, so a send through here always records what it knows.
+func carriedBuilds(modules []string, leftOut map[string]string, current map[string]inventory.CurrentBuild,
+	before map[string]string) map[string]string {
+	out := map[string]string{}
+	for _, m := range modules {
+		if _, left := leftOut[m]; left {
+			if was, kept := before[m]; kept {
+				out[m] = was
+			}
+			continue
+		}
+		out[m] = current[m].Commit
+	}
+	return out
 }
 
 // sortedKeysOf is a map's keys, sorted — so what a declaration says it left out does not move