Answer held and say artifacts-gone, so the builder rebuilds a done key whose blob the artifact store no longer holds (issue 1937)

A done key the controller never took in holds no manifest, and the store's nightly collector deleted its bare blob:
node-tools', mesh-host's and the forge bridge's keys name blobs that answer 404. Since issue 1933 the take-in refuses such a
key, and nothing built it again. held (in the serving process, HEADs only) lets the builder ask before it reuses a key; a
refused take-in now names every gone archive and says artifacts-gone with the module, key, fingerprint and references.

internal/link/events.go

@@ -89,6 +89,9 @@ const (
 	KeyDeclaredConfigChanged = "declared-config-changed"
 	// KeyPredefinedChanged: the predefined-keys mirror's one key was written (design 57 K3): a newer list kept.
 	KeyPredefinedChanged = "predefined-changed"
+	// KeyArtifactsGone: a builder's key was not taken in because the artifact store does not hold an artifact it names
+	// (novox/hq issue 1937): the module, the key, its fingerprint and every gone reference. The builder builds the key again.
+	KeyArtifactsGone = "artifacts-gone"
 )
 
 // Applied is what a machine now runs, as the mesh states it.