When the player starts casting Hearthstone (spell IDs 6948/8690),
trigger background terrain loading at the bind point so tiles are
ready when the teleport fires.
- Add HearthstonePreloadCallback to GameHandler, called from
handleSpellStart when a Hearthstone cast begins.
- Application callback enqueues a 5×5 tile grid around the bind
point via precacheTiles() (same-map) or starts a file-cache warm
via startWorldPreload() (cross-map) during the ~10 s cast time.
- On same-map teleport arrival, call processAllReadyTiles() to
GPU-upload any tiles that finished parsing during the cast before
the first frame at the new position.
Fixes: player landing in unloaded terrain and falling after Hearthstone.