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/compose_sdk.go

@@ -200,6 +200,7 @@ func composeQuestion(ctx context.Context, open *stores, node string, mode compos
 	}
 	sayFound(ctx, d)
 	out := sendableOf(d)
+	out.fileReferences = readsFileReferences(in.Input, node)
 	if !withOrder {
 		out.Sequence, out.Epoch, out.Generation = 0, 0, 0
 	}
@@ -281,6 +282,7 @@ func composeSendValidated(ctx context.Context, open *stores, node string) (senda
 		if len(left) == 0 && len(in.wouldMake) == 0 {
 			sayFound(ctx, d)
 			out := sendableOf(d)
+			out.fileReferences = readsFileReferences(in.Input, node)
 			// The sender stamps the order it allots (order.stamp); a composition for a send carries none of its own.
 			out.Sequence, out.Epoch, out.Generation = 0, 0, 0
 			here, _ := nodeOf(in.Input, node)