Lead a refusal of a held module's stand-in with why the module was held, so its first reason is never hidden (issue 774)

The proposal on ace said only "mesh-self-check runs as its own account and declares no own secret broker": the words
of its bare stub, held for "has no bus credential", which no one saw. The second hidden first reason of the day, after
"0 bundle(s)".

Quick fix: log the held map in composeEach. Thorough fix: firstReason leads a refusal no culprit is found for with the
first reason of every held module it names ("<m> is held: <why>; its stand-in is then refused: <err>"), for Compose and
the node's Answer alike; the hold's own why already keeps its first words. Chose thorough.

go/compose/frombus/answer.go

@@ -335,7 +335,7 @@ func composeEach(in compose.Input, node string, fps map[string]string, applied b
 		if err != nil {
 			module, leave := culprit(in, node, fps, mode, failed, err)
 			if module == "" {
-				return nil, nil, nil, err
+				return nil, nil, nil, firstReason(err, failed)
 			}
 			hold(module, err.Error(), leave)
 			continue
@@ -373,6 +373,23 @@ func composeEach(in compose.Input, node string, fps map[string]string, applied b
 	return out, skipped, stood, nil
 }
 
+// firstReason is err, a refusal no culprit is found for, led by the first reason of each held module it names: a held
+// module's stand-in refused later never replaces why it was held (issue 774, 2026-10-30: mesh-self-check on ace held for
+// "has no bus credential", then refused as "runs as its own account and declares no own secret broker", its bare stub's
+// words, which were all the proposal's refusal said).
+func firstReason(err error, failed map[string]string) error {
+	var named []string
+	for _, m := range sortedKeys(failed) {
+		if strings.Contains(err.Error(), m) {
+			named = append(named, m+" is held: "+failed[m])
+		}
+	}
+	if len(named) == 0 {
+		return err
+	}
+	return fmt.Errorf("%s; its stand-in is then refused: %w", strings.Join(named, "; "), err)
+}
+
 // blames says the composition with m's stub (again) makes m the culprit of err: it composes, or fails otherwise. The
 // runtime's stub is never a runtime (no build is taken in, so it carries no bundle) and always fails otherwise: it is
 // the culprit only where its stub composes (issue 774, 2026-10-30 19:30: node-tools held for any error naming it).

go/compose/frombus/frombus_test.go

@@ -142,6 +142,7 @@ func TestInput(t *testing.T) {
 	t.Run("a node refused whole says what it held", aNodeRefusedWholeSaysWhatItHeld)
 	t.Run("the runtime is blamed only where its stub composes", theRuntimeIsBlamedOnlyWhereItsStubComposes)
 	t.Run("a running module waits for its build", aRunningModuleWaitsForItsBuild)
+	t.Run("a held module's first reason leads", aHeldModulesFirstReasonLeads)
 	t.Run("another node not resolved is named", otherNodeNotResolvedIsNamed)
 	ctx := context.Background()
 	xterm, err := os.ReadFile("../../catalogue/testdata/catalogue-835bf22b-xterm.module.json")

go/compose/frombus/held_rows_test.go

@@ -131,3 +131,23 @@ func rawStrings(rs []json.RawMessage) []string {
 	}
 	return out
 }
+
+// Issue 774: a refusal of a held module's stand-in leads with why the module was held, its first reason, as mesh-self-check's
+// on ace should have; a refusal naming no held module is as it was.
+func aHeldModulesFirstReasonLeads(t *testing.T) {
+	held := map[string]string{"mesh-self-check": "mesh-self-check on ace has no bus credential: nothing was issued for ace.mesh-self-check"}
+	for _, row := range []struct {
+		err, want string
+	}{
+		{"mesh-self-check runs as its own account and declares no own secret broker",
+			"mesh-self-check is held: mesh-self-check on ace has no bus credential: nothing was issued for ace.mesh-self-check; " +
+				"its stand-in is then refused: mesh-self-check runs as its own account and declares no own secret broker"},
+		{"ace is the hub and has no endpoint", "ace is the hub and has no endpoint"},
+	} {
+		err := errors.New(row.err)
+		got := firstReason(err, held)
+		if got.Error() != row.want || !errors.Is(got, err) {
+			t.Errorf("refused %q, want %q", got, row.want)
+		}
+	}
+}