Send files over 64 KiB by digest through the bus's object bucket to nodes that read them, grant it, and warn at 75 percent (issues 1696, 1697, 1698; ADR 0486 step 3)

A node that reports file-references gets the digest and size of each file over 64 KiB that may travel so, and the content is put in
the bucket declaration-files before the declaration is sent; others get every content inline. The controller alone writes the
bucket and node-engines read an object by name (the four grants the node-engine's own fetch makes, no list, no write). An object
goes a day after no node's latest declaration names it. The near-limit condition warns at 75 percent, and a test holds the control
node's declaration, in the reference form, under half the bus's largest message.

cmd/mesh-controller/sendable.go

@@ -39,6 +39,10 @@ type sendable struct {
 	// ReadsModules says the machine's node-engine reads a module per resource and a build per module (novox/hq issue
 	// 1145): only then are they sent, because an older node-engine refuses a key it does not know, whole.
 	ReadsModules bool
+	// fileReferences says the machine's node-engine reports the capability of reading a file given as a digest and a size
+	// (declaration.FileReferencesCapability, novox/hq ADR 0486 §5): then every file over 64 KiB that may travel so does (Body,
+	// objects). Not on the wire as a field; the bodies differ.
+	fileReferences bool
 	// moduleOf is the composer's owner of each resource id, and buildIDs the build each module carries (its id):
 	// kept beside the resources, and on the wire only to a machine that reads them.
 	moduleOf map[string]string
@@ -115,12 +119,16 @@ type adoptionEnvelope struct {
 
 // Body is the declaration's bytes, as sent and as digested.
 func (s sendable) Body() ([]byte, error) {
-	envelope := map[string]any{"declaration": 1, "resources": s.Resources}
+	resources, _, err := s.byReference()
+	if err != nil {
+		return nil, err
+	}
+	envelope := map[string]any{"declaration": 1, "resources": resources}
 	if s.ReadsModules {
 		// The module of each resource the composer owns (what the mesh adds of its own has none), and the build of each
 		// module (novox/hq issue 1145): to a machine that said it reads them, and no other.
-		named := make([]map[string]any, len(s.Resources))
-		for n, r := range s.Resources {
+		named := make([]map[string]any, len(resources))
+		for n, r := range resources {
 			module, owned := s.moduleOf[fmt.Sprint(r["id"])]
 			if !owned {
 				named[n] = r
@@ -165,3 +173,19 @@ func (s sendable) Body() ([]byte, error) {
 	}
 	return json.Marshal(envelope)
 }
+
+// byReference is the resources as the machine is sent them: with the contents of its large files replaced by their digests when
+// it reports it reads them (issue 1696), and the contents it replaced, by the name of the object that holds each.
+func (s sendable) byReference() ([]map[string]any, map[string][]byte, error) {
+	if !s.fileReferences {
+		return s.Resources, nil, nil
+	}
+	return referencing(s.Resources)
+}
+
+// objects are the contents the body names by digest, by the name of each one's object: what is put in the bucket before the
+// body is sent. Empty for a machine that is sent every content inline.
+func (s sendable) objects() (map[string][]byte, error) {
+	_, objects, err := s.byReference()
+	return objects, err
+}