fix(director): discard an asset cancelled during reader cleanup

The finally block awaits reader.cancel(), so a cancel arriving there still let
the bytes return. Rechecks the signal after cleanup, with a test that keeps the
caller's abort reason, and wraps the current-state note.

docs/CURRENT-STATE.md

@@ -396,7 +396,9 @@ and geographic placement remain distinct; Stop/replacement releases resources.
 Import acquires nothing. See [data packs](DIRECTOR-DATA-PACKS.md). Existing scene
 content, assets and credit remain unchanged.
 
-The asset directory source rejects with the caller's abort reason if cancellation arrives during any read, including the final one.
+The asset directory source rejects with the caller's abort reason if
+cancellation arrives during any read, including the final one, or while the
+reader is being released.
 
 Director version-4 documents add named geographic anchors and optional explicit
 camera moves with easing, duration and holds. Playback and seek share one