Leave a module out of a walk when its check built it the same as what runs (hq issue 721)

A one-sentence change in a package mesh-builder imports but never reaches moved
build-agent. A merge check now compares each module it builds with the build the
mesh holds, the whole image without file times, labels or creation time, and a
walk is cut without a module every check of its pull requests found the same,
while the mesh still holds that build. A comparison that fails moves the module
as before.

cmd/mesh-controller/batches.go

@@ -1000,10 +1000,15 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	plan.Delivery.Merges = named
 	plan.Delivery.Alone = batch.Delivery != nil && batch.Delivery.Alone
 	plan.Delivery.Withheld = withheld
+	// **Left where it is, a module whose program did not change** (novox/hq issue 721): its check found it the same as
+	// the build the mesh holds.
+	for n, why := range leaveSameAsRunning(ctx, inv, &plan) {
+		unchanged[n] = why
+	}
 	plan.Delivery.Unchanged = unchangedLeft(unchanged, plan.Modules)
 	// **Its moments and its class** (novox/hq ADR 0282 decision 6): measured, never acted on.
 	plan.Times = walkTimesAtCut(*batch, plan, entries, now)
-	if len(moved) == 0 {
+	if len(moved) == 0 || len(plan.Modules) == 0 {
 		plan.State = inventory.PlanDone
 		plan.Tiers = [][]string{}
 		plan.Note = "moved nothing the mesh holds: nothing to walk"