Keep the builds every want names and each composition's provisioner plans; find the replaced controller in the build records (issue 1945)

With no plan left to name a build or a send to keep the plans, a want names what a collection must keep, and the
composition is where the provisioner's plans are composed for the assignments mirror (ADR 0539 clauses 1 and 5);
the assignments no longer name a node's build, so the controller replaced is the newest earlier build of it.

cmd/mesh-controller/composed_plans_test.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"encoding/json"
+	"testing"
+
+	"git.novox.be/novox/mesh-sdk/go/catalogue"
+)
+
+// 1945 (dev-r6-controller's finding): the provisioner's plans a composition hands are kept in the shape the
+// assignments mirror reads (inventory.SentProvisioningPlans: each module's plan and what requires it), and a
+// composition that hands none keeps none.
+func TestAComposedNodesProvisionerPlansAreKept(t *testing.T) {
+	if plansKept(sendable{}) != nil {
+		t.Errorf("a composition that hands nothing kept plans")
+	}
+	d := sendable{plans: map[string]catalogue.Plan{"umami": {Placement: map[string]string{"dir:state": "/var/lib/umami"}}},
+		requiredBy: map[string]string{"umami": "analytics"}}
+	var kept map[string]struct {
+		Placement  map[string]string `json:"placement"`
+		RequiredBy string            `json:"required-by"`
+	}
+	if err := json.Unmarshal(plansKept(d), &kept); err != nil {
+		t.Fatal(err)
+	}
+	if kept["umami"].Placement["dir:state"] != "/var/lib/umami" || kept["umami"].RequiredBy != "analytics" {
+		t.Errorf("kept %+v", kept)
+	}
+}