Keep the seat mesh-placement's state as one key per module and node, so a node watches its own keys and a change writes only the modules it moves (issue 774, research 072 row 37)

The node key <node>.node is a write's commit: it names each placed module with its value's digest, the
module keys carry the value they replace until it lands, so a reader never composes half a write. Every
reader goes through PlacementKeys/PlacementsOf, which read the legacy placement.<node> too while the
change is delivered.

go/compose/frombus/frombus_test.go

@@ -819,10 +819,11 @@ func statesOf(t *testing.T, ctx context.Context, b *bus) mapStates {
 		out[state][key] = raw
 	}
 	placements, _ := b.Placements(ctx)
-	for node, p := range placements {
-		k, _ := delivery.PlacementKey(node)
-		raw, err := delivery.EncodePlacement(p)
-		put(PlacementRead, k, raw, err)
+	for _, p := range placements {
+		values, err := delivery.PlacementValues(p)
+		for k, raw := range values {
+			put(PlacementRead, k, raw, err)
+		}
 	}
 	records, _ := b.Records(ctx)
 	for node, r := range records {

go/compose/frombus/states.go

@@ -68,15 +68,11 @@ func (r statesReader) each(read string, visit func(key string, value []byte)) er
 }
 
 func (r statesReader) Placements(context.Context) (map[string]delivery.Placement, error) {
-	out := map[string]delivery.Placement{}
+	keys := delivery.NewPlacementKeys()
 	err := r.each(PlacementRead, func(k string, v []byte) {
-		if node, err := delivery.ParsePlacementKey(k); err == nil {
-			if p, err := delivery.DecodePlacement(v); err == nil && p.Node == node {
-				out[node] = p
-			}
-		}
+		_, _ = keys.Put(k, v, 0, false) // a key that does not read places nothing, as before
 	})
-	return out, err
+	return keys.All(), err
 }
 
 func (r statesReader) Records(context.Context) (map[string]delivery.NodeRecord, error) {