Compare a build with the one the mesh registered, not the newest, which may be one put back (review of #336, hq issue 721)

internal/link/build.go

@@ -182,6 +182,11 @@ type CheckRequest struct {
 	// each, from the merge it judged, into the scratch namespace once it has judged and not failed, for the walk to
 	// take. A build seat from before it builds nothing, and the walk builds as before.
 	Builds []CheckBuild `json:"builds,omitempty"`
+	// Running is, keyed "<module>/<artifact>", each artifact of the build the mesh has registered for its module — the
+	// build its machines are sent (novox/hq issue 721): what a build for the merge is compared with, never the newest
+	// build, which may be one its gate failed and the mesh put back. A module absent is compared with nothing, and
+	// moves. A controller from before it sends none, and nothing is said unchanged.
+	Running map[string]string `json:"running,omitempty"`
 	// Named are the builds the seat's verdicts of earlier checks of this pull request at this head name, within the
 	// week check builds are kept (novox/hq issue 703): asked again because only the trunk moved, the check keeps a
 	// build kept under a module's unchanged key, instead of building it again, only when one of these names its key