Close three holes in the card release path

A second review pass over the pool. All three are the same shape: state
or a listener outliving the video it belonged to.

attachProxyFallback replaces whatever fallback an image currently has,
so a call arriving late -- a race hitting its patience timeout after the
card was recycled -- took away the live listener and left a dead one, and
the new video's thumbnail would then fail with nothing behind it. This
one was self-inflicted: the detach came in last round to stop the
listeners accumulating, and introduced the clobber. It is token-guarded
now, like every other path that can arrive late.

The favourite pop is cleared by animationend, which never fires on a card
release() has already detached -- detached elements run no animations. So
the class rode into the pool and replayed on the next video the card
showed. Favourite something and flick-scroll to see it.

And withOrigin compared a token that dataset reports as undefined for an
unstamped card, which matched every unstamped card instead of none --
failing open in the guard whose whole purpose is noticing that the grid
has recycled the card out from under the player.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QBDkEXP4htyXTCZUwMLphd

tests/smoke_grid.py

@@ -61,6 +61,7 @@ INSPECT = """() => {
             heart_shown: fav ? fav.classList.contains('is-favorite') : null,
             heart_expected: v ? App.favorites.has(v) : null,
             stale_loading: card.classList.contains('is-loading'),
+            stale_pop: fav ? fav.classList.contains('just-favorited') : false,
         };
     });
 }"""
@@ -158,6 +159,13 @@ def check_cards(c, cards, phase):
     c.ok(f"{phase}: no card left in the loading state", not stale,
          f"{len(stale)} stuck" if stale else "")
 
+    # The favourite pop is animated away by animationend, which never fires on a
+    # card released mid-animation -- so it can ride into the pool and replay on
+    # whatever video the card is bound to next.
+    popping = [x for x in cards if x["stale_pop"]]
+    c.ok(f"{phase}: no card replaying the favourite animation", not popping,
+         f"{len(popping)} popping, e.g. id={popping[0]['id']}" if popping else "")
+
 
 def main():
     c = Checks()