A restored node whose source run is gone says its curve was lost

The run detail works out curve_lost per node when read, from whether its
cached_from run still exists. The run page names the deleted run in place
of the generic NO_CURVE note.

docs/concepts/runs.md

@@ -143,7 +143,8 @@ missing artifact bytes make an entry a miss.
 
 A cache hit does not replay emissions. Its metrics are read from the original
 run, so comparisons still show the recorded curve. Deleting that original run
-leaves cached results available but removes their curve.
+leaves cached results available but removes their curve. The run's page then
+names the deleted run each lost curve went with.
 
 ## Objects that cannot be serialized