Un-key stage-cache rows written before the current key shape

CACHE_SHAPE_SINCE marks the last change that moved every cache key. The
run service's periodic sweep clears cache_key on rows of runs created
before it, which nothing can match any more. Their outputs stay.

docs/concepts/runs.md

@@ -139,7 +139,9 @@ Use `fluksio run ... --no-cache`, `fluksio sweep ... --no-cache`, or
 
 Only Python nodes are cached. Built-in and connector nodes are not. The cache
 uses run history; outputs too large to store there are not cacheable, and
-missing artifact bytes make an entry a miss.
+missing artifact bytes make an entry a miss. An entry written before the cache
+key last changed shape can never match again, so the engine drops it from the
+cache and keeps the node's recorded outputs.
 
 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