Fix swap animation stutter and remove 1s server-side dead delay

- Remove unused card_revealed broadcast + 1s asyncio.sleep in swap handler
  (client never handled this message, causing pure dead wait before game_state)
- Defer swap-out (opacity:0) on hand cards to onStart callback so overlay
  covers the card before hiding it — eliminates visual gap for all players
- Defer heldCardFloating visibility hide to onStart — held card stays visible
  until animation overlay replaces it
- Thread onStart callback through animateUnifiedSwap → _runUnifiedSwap

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/handlers.py

@@ -313,22 +313,6 @@ async def handle_swap(data: dict, ctx: ConnectionContext, *, broadcast_game_stat
                     reason=f"swapped {drawn_card.rank.value} into position {position}, replaced {old_rank}",
                 )
 
-            # Broadcast reveal of old face-down card before state update
-            if old_card_data:
-                reveal_msg = {
-                    "type": "card_revealed",
-                    "player_id": ctx.player_id,
-                    "position": position,
-                    "card": old_card_data,
-                }
-                for pid, p in ctx.current_room.players.items():
-                    if not p.is_cpu and p.websocket:
-                        try:
-                            await p.websocket.send_json(reveal_msg)
-                        except Exception:
-                            pass
-                await asyncio.sleep(1.0)
-
             await broadcast_game_state(ctx.current_room)
             await asyncio.sleep(1.0)
             check_and_run_cpu_turn(ctx.current_room)