Say an act's answer from one resolution after its write (issue 237)

assign read the whole mesh before its write, to record the seats held by
derivation, and that read printed on standard error that the node did not
resolve for lack of the seat the act was about to give it. The seat verb
answers with both streams, so the stale line came last and named the
assignment just made as its remedy.

The reads inside an act no longer print 'left out' lines; the node's
unmet seat dependencies and whether it resolves come from one planFor
taken after the write, and every other machine from blockedElsewhere.
unassign now says when its node does not resolve, from the same
resolution.

cmd/mesh-controller/plan.go

@@ -292,7 +292,7 @@ func theRestOfTheMesh(ctx context.Context, inv *inventory.Inventory,
 		if err != nil {
 			// Said, not skipped: a machine dropped here offers nothing and holds nothing as far
 			// as every other machine's plan can tell (novox/hq issue 188).
-			fmt.Fprintf(os.Stderr, "%s is left out of the rest of the mesh: it does not resolve: %v\n", o.node.Name, err)
+			sayLeftOut(ctx, o.node.Name, err)
 			continue
 		}
 		firstHeld = append(firstHeld, got.Claims...)
@@ -337,7 +337,7 @@ func theRestOfTheMesh(ctx context.Context, inv *inventory.Inventory,
 			// resolved without its offers, and one that consumes them cannot resolve here by
 			// design — that is not the machine being dropped, it is the view being partial.
 			if exclude == "" {
-				fmt.Fprintf(os.Stderr, "%s is left out of the rest of the mesh: it does not resolve: %v\n", o.node.Name, err)
+				sayLeftOut(ctx, o.node.Name, err)
 			}
 			continue
 		}
@@ -347,6 +347,35 @@ func theRestOfTheMesh(ctx context.Context, inv *inventory.Inventory,
 	return world, nil
 }
 
+// leftOutKey carries where theRestOfTheMesh says a machine it leaves out, for an act that answers with
+// its own resolution instead (novox/hq issue 237).
+type leftOutKey struct{}
+
+// leftOutSaidBy is ctx with theRestOfTheMesh's "left out" lines written to w rather than to standard
+// error.
+//
+// **An act's answer is one resolution taken after its write** (novox/hq issue 237, decided 2026-10-11).
+// `assign` read the whole mesh once before its write — to record the seats held by derivation — and that
+// read said, on standard error, that the node did not resolve for lack of the seat the act was about to
+// give it. The controller seat's verb answers with both streams, so the stale line came last, after the
+// act's own "now has it held", and named as its remedy the assignment just made. An act gives io.Discard
+// here: what the node resolves to after the write is said by its own planFor, and every other machine
+// that does not resolve by blockedElsewhere, both read after the write.
+func leftOutSaidBy(ctx context.Context, w io.Writer) context.Context {
+	return context.WithValue(ctx, leftOutKey{}, w)
+}
+
+// sayLeftOut says that a machine is left out of the rest of the mesh, where ctx says to: standard
+// error unless an act said otherwise. Said, not skipped: a machine dropped there offers nothing and
+// holds nothing as far as every other machine's plan can tell (novox/hq issue 188).
+func sayLeftOut(ctx context.Context, node string, err error) {
+	w, ok := ctx.Value(leftOutKey{}).(io.Writer)
+	if !ok {
+		w = os.Stderr
+	}
+	fmt.Fprintf(w, "%s is left out of the rest of the mesh: it does not resolve: %v\n", node, err)
+}
+
 // declarationFor is everything a node would be sent.
 //
 // One place, because there were three and one of them was written before credentials existed and
@@ -1751,28 +1780,43 @@ func (l *unheldLogger) log(node string, unheld []catalogue.Unheld) {
 // which includes every one of a module just assigned — and each now met that was not. Nothing about
 // any other node, and nothing that was already true before the act.
 func unheldChange(shelf map[string]catalogue.Manifest, node string, before, after []string) []string {
-	judge := func(names []string) map[string]catalogue.Unheld {
-		var set []catalogue.Manifest
-		for _, n := range names {
-			if m, known := shelf[n]; known {
-				set = append(set, m)
-			}
+	return unheldChangeTo(shelf, node, before, after, unheldOver(shelf, node, after))
+}
+
+// unheldOver is the node's unmet seat dependencies judged over a set of module names alone.
+func unheldOver(shelf map[string]catalogue.Manifest, node string, names []string) []catalogue.Unheld {
+	var set []catalogue.Manifest
+	for _, n := range names {
+		if m, known := shelf[n]; known {
+			set = append(set, m)
 		}
-		out := map[string]catalogue.Unheld{}
-		for _, u := range catalogue.UnheldDependencies(shelf, node, set, nil) {
-			out[u.Module+" "+u.Seat] = u
-		}
-		return out
 	}
-	was, now := judge(before), judge(after)
+	return catalogue.UnheldDependencies(shelf, node, set, nil)
+}
+
+// unheldChangeTo is unheldChange with the node's dependencies as they now stand given: `now` is what
+// the act's one resolution after its write says is unmet, and `after` the modules that resolution has
+// on the node (novox/hq issue 237). A line about how things stand after the act is never judged from
+// anything else, so it cannot disagree with `plan` for the node.
+func unheldChangeTo(shelf map[string]catalogue.Manifest, node string, before, after []string,
+	now []catalogue.Unheld) []string {
+	key := func(u catalogue.Unheld) string { return u.Module + " " + u.Seat }
+	was := map[string]catalogue.Unheld{}
+	for _, u := range unheldOver(shelf, node, before) {
+		was[key(u)] = u
+	}
+	is := map[string]catalogue.Unheld{}
+	for _, u := range now {
+		is[key(u)] = u
+	}
 	var lines []string
-	for _, k := range sortedNames(now) {
+	for _, k := range sortedNames(is) {
 		if _, already := was[k]; !already {
-			lines = append(lines, "but "+now[k].String())
+			lines = append(lines, "but "+is[k].String())
 		}
 	}
 	for _, k := range sortedNames(was) {
-		if _, still := now[k]; still {
+		if _, still := is[k]; still {
 			continue
 		}
 		u := was[k]