Record the bases a build copies, keep them by the builds that stood on them, and copy each image once

A copied base was named only in what a build stood on, and nowhere when the build failed,
so the store's sweep could never let one go (hq issue 321). One repository per upstream
image stops each module asking the public registry for the same image again, and letting
an index go now takes its own platform manifests, which otherwise kept every byte. A
person can record the copies no record names through the new mirrors verb (hq ADR 0257).

The forge test fix is the same commit as on feat/plain-notifications: main fails without it.

cmd/mesh-builder/main.go

@@ -264,6 +264,8 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 			request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
 			forgeFrom(), say, request.Seats)
 	}
+	// What it copied into the store, whatever became of the build (novox/hq ADR 0257).
+	result.Mirrored = built.Mirrored
 	// Only a build the kill ended: the kill came before its work did. One that finished — built, or
 	// failed on its own — in the moment the kill arrived says what it did, and the kill is refused.
 	killed := false