Make legacy full revalidations visible to reads
Full responses were excluded from the canonical re-key on the assumption
that full entries are always spawned at their final, concrete path, so the
re-key would be a no-op. That holds for a full prefetch into an empty slot,
but a full prefetch that revalidates an existing entry spawns into the
revalidation slot, and the re-key is the only thing that moves it out. Its
response stayed invisible to reads while the stale entry kept serving. For
entries spawned at the canonical path the re-key is an in-place replace at
the same concrete key, so dropping the exclusion costs nothing.
