A held module is no dependence of its neighbours: its hold stops spreading (issue 1126)

novox's seed p-20261028t231141-ec84b4 refused 11 modules "what it composes of ..., held, is not what runs of them":
de-spiegel's hold spread through mailu to postgres, nftables and route-proxy, and from postgres to every consumer, and
gitea, held so, refused the node: "its credential of postgres-database is handed to the provisioner and the module is
not: nothing would fill it". Three rounds each moved the error. The kind: a held module still took part in the
composition as a dependence, so its hold spread.

- The quick fix: let heldAsToday also follow dependsOnHeld's words, and skip the handed refusal for gitea. It moves the
  error once more and leaves every node's applying path holding consumers.
- The thorough fix, chosen: a held module composes only as what runs of it. composeEach no longer compares the
  composition with each held module as its stub (dependsOnHeld removed): a consumer or collector composes against what
  runs of the held module like any other module, and a real difference is still a named difference in the answer, a
  module's own refusal still its refusal. A stub (compose.Stub, catalogue.Manifest.Stubbed) leaves the credentials and
  own secrets it would hand the provisioner out of its plan as they run, never refused for want of a plan. Only what a
  held module's new manifest says to others in their own files is still held behind it (dependents: the provider of
  what it contributes to, the filter where it listens), which heldAsToday already follows.

Tests: rows added to existing tests (a held provider with a binding consumer, which the old code held in the seed's
words; a held feeder of a collector; a held module with a handed credential and own secret). No new test function.

go/compose/frombus/frombus_test.go

@@ -45,6 +45,8 @@ type bus struct {
 	served     map[string]map[string]any
 	servedRead []string
 	buildsRead []string
+	// held answers every credential asked as held, sealed to the consumer.
+	held bool
 }
 
 func (b *bus) Build(_ context.Context, module, fp string) (*delivery.BuilderBuild, error) {
@@ -89,7 +91,7 @@ func (b *bus) OwnSecrets(context.Context, string) ([]compose.OwnSecretRow, error
 	return b.own, b.ownErr
 }
 
-func (b *bus) Credentials(_ context.Context, node string, asConsumer, _ []compose.CredentialKey) ([]compose.Credential, []compose.Issued, error) {
+func (b *bus) Credentials(_ context.Context, node string, asConsumer, asProvider []compose.CredentialKey) ([]compose.Credential, []compose.Issued, error) {
 	b.asked = append(b.asked, asConsumer...)
 	for _, k := range asConsumer {
 		placed := false
@@ -100,7 +102,17 @@ func (b *bus) Credentials(_ context.Context, node string, asConsumer, _ []compos
 			return nil, nil, fmt.Errorf("%s is not on %s: neither assigned there nor required by a module there", k.ConsumerModule, node)
 		}
 	}
-	return nil, nil, nil
+	var out []compose.Credential
+	var issued []compose.Issued
+	for _, k := range append(append([]compose.CredentialKey{}, asConsumer...), asProvider...) {
+		if b.held {
+			out = append(out, compose.Credential{Provision: k.Provision, Consumer: k.Consumer, ConsumerModule: k.ConsumerModule,
+				Provider: k.Provider, Local: k.Local, State: compose.CredentialHeld, Sealed: `"box"`})
+			issued = append(issued, compose.Issued{Provision: k.Provision, Consumer: k.Consumer, ConsumerModule: k.ConsumerModule,
+				Provider: k.Provider, Local: k.Local, ForProvider: `"box"`})
+		}
+	}
+	return out, issued, nil
 }
 
 func (b *bus) Values(_ context.Context, node, module string) (map[string]any, error) {