Add set_destination remote interface to player_sync

A plugin that moves a player calls remote.call("player_sync",
"set_destination", player_name, instance_id) before connect_to_server.
It only records the destination. Every leave clears it, and a leave
that uploads with reason switching_servers adds it to the upload as
destination_instance_id. The README describes it for plugin authors.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

plugins/player_sync/module/serialize.lua

@@ -959,6 +959,7 @@ end
 --- @field personal_logistic_slots table?
 --- @field crafting_queue table?
 --- @field recipe_notifications string?
+--- @field destination_instance_id integer? Only on a leave upload, for instance.ts, which strips it
 
 --- @param player LuaPlayer
 --- @return table<string, boolean>