Read a rebuild of an unchanged source as no move, whatever image digest it made (hq issue 280)

An image is not byte-reproducible, so ADR 0236's 'same artifacts is no move'
never held for one: a catalogue merge that did not touch the bus rebuilt it,
and every send to the control node waited for a planned bus upgrade.

The builder now records a source fingerprint per build (module tree, context
trees, bases and toolchains by digest). A rebuild with the fingerprint of the
build it repeats is registered with that build's artifacts, handed to modules
standing on it, holds no push, demands no bus step, and a plan sends and
gates nothing for it. Identical artifacts remain a second way to be no move.

cmd/mesh-controller/bus_step.go

@@ -54,13 +54,15 @@ type busPending struct {
 	machines []string
 	from     map[string]string
 	to       string
-	// same are the commits whose build made the same artifacts and manifest as the build the mesh holds.
+	// same are the commits whose build was made from the same source as the build the mesh holds, or
+	// made the same artifacts and manifest (novox/hq issue 280).
 	same map[string]bool
 }
 
 // moves is whether sending the machine would replace its bus: a build it was not last sent, unless the
-// two builds made the same artifacts from the same manifest — a rebuild of the same source for another
-// module's merge changes nothing the machine runs.
+// two builds were made from the same source, or made the same artifacts from the same manifest — a
+// rebuild of the same source for another module's merge changes nothing the machine runs, whatever
+// image digest it made (novox/hq issue 280).
 func (b busPending) moves(machine string) bool {
 	from, known := b.from[machine]
 	if b.module == "" || b.to == "" || (known && sameCommit(from, b.to)) {
@@ -100,6 +102,15 @@ func pendingBus(ctx context.Context, inv *inventory.Inventory) (busPending, erro
 	for commit, refs := range made {
 		b.same[commit] = refs != "" && refs == made[b.to]
 	}
+	sources, err := inv.BuildSourceFingerprints(ctx, b.module)
+	if err != nil {
+		return b, err
+	}
+	for commit, src := range sources {
+		if src != "" && src == sources[b.to] {
+			b.same[commit] = true
+		}
+	}
 	for _, n := range b.machines {
 		sent, known, err := inv.SentBuilds(ctx, n)
 		if err != nil {