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.

internal/link/declare.go

@@ -41,7 +41,7 @@ func Declare(ctx context.Context, bus Bus, signer Signer, node string,
 		return fmt.Errorf("cannot sign a declaration for %s: %w", node, err)
 	}
 
-	body, err := json.Marshal(Signed{Declaration: declaration, Signature: signature})
+	body, err := Envelope(declaration, signature)
 	if err != nil {
 		return err
 	}
@@ -66,8 +66,16 @@ func Declare(ctx context.Context, bus Bus, signer Signer, node string,
 	return bus.PublishDeclaration(publish, node, body)
 }
 
-// NearTheLimit is the share of the bus's largest message at which a declaration is said to be near it.
-const NearTheLimit = 0.9
+// Envelope is the message a declaration goes on the bus as: the declaration and its signature, each as base64 in one JSON
+// document. Its length is what the bus's largest message bounds.
+func Envelope(declaration, signature []byte) ([]byte, error) {
+	return json.Marshal(Signed{Declaration: declaration, Signature: signature})
+}
+
+// NearTheLimit is the share of the bus's largest message at which a declaration is said to be near it: three quarters, so that
+// a declaration which should stay far from the limit raises the condition while there is still room to act (novox/hq ADR 0486
+// §8; the first fix of issue 1689 warned at nine tenths while every file was still inline).
+const NearTheLimit = 0.75
 
 // DeclarationMeasured is told, at every send of a declaration, its size on the wire and the bus's largest message,
 // so the controller raises a condition when it nears the limit and clears it when it is back under (issue 1689).