The epoch check ran only after _dispatch() returned, by which point the
capture path had already written the PNG to the media cache, spilled the
element tree to disk and routed the frame through auxiliary vision — the
human's screen had left the process before being "discarded". A fence
callable is threaded into _dispatch; capture and capture_after call it
the moment backend.capture() returns, and the post-dispatch check stays
for every other action.
(cherry picked from commit 70c09e5fad89d2817f13aea772bd845ab7cc4c8a)
12deaf935f