Take no rebuild into a waiting walk, and ask what a tier left unasked (hq issue 754)

A rebuild that landed while a walk waited for mesh-delivery's word was taken as
the walk's build of that module; once let go, the walk asked a tier only when
every module of it was unasked, so it never asked the rest and stood still.

A walk now takes only the outcomes of builds it asked, and none while it waits;
a rebuild does not join a waiting walk and says it runs beside it; and a tier
asked in part is asked the rest, including a module held built that the walk
never asked for, which is somebody else's build and not sent as the walk's.

cmd/mesh-controller/queue.go

@@ -521,7 +521,9 @@ func rebuildCommand(ctx context.Context, args []string) error {
 	}
 
 	// **A module a plan holds unbuilt, or failed, joins that plan** rather than running beside it: the
-	// plan would ask it again, or stay failed on an outcome a rebuild has replaced.
+	// plan would ask it again, or stay failed on an outcome a rebuild has replaced. A walk waiting for its word
+	// is not joined (novox/hq issue 754): the rebuild runs beside it, and says so.
+	var beside string
 	if module != "" {
 		joined, said, err := joinAPlan(ctx, open, module)
 		if err != nil {
@@ -531,6 +533,7 @@ func rebuildCommand(ctx context.Context, args []string) error {
 			fmt.Println(said)
 			return nil
 		}
+		beside = said
 	}
 	id, err := askABuild(ctx, source, path, ref)
 	if err != nil {
@@ -538,6 +541,10 @@ func rebuildCommand(ctx context.Context, args []string) error {
 	}
 	recordAsked(ctx, inventory.BuildRequest{ID: id, Repository: source.Repository, Seat: source.Seat, Path: path,
 		Ref: ref, For: "rebuild"})
+	if beside != "" {
+		fmt.Printf("rebuild asked as %s, beside the walk: %s\n", id, beside)
+		return nil
+	}
 	fmt.Printf("rebuild asked as %s\n", id)
 	return nil
 }