feat(pack): ship offline usernamecache to clients for Jade

Jade resolves tamed-animal owner names client-side; its Mojang profile
lookup can't resolve offline-mode UUIDs and it falls back to NeoForge's
UsernameCache, which the client loads from <game>/usernamecache.json.
Ship that file (side = "client", installed at the game root) so migrated
owners stop showing as "???".

Maintain the mapping with tools/sync-usernamecache.py after new logins.

usernamecache.pw.toml

@@ -0,0 +1,8 @@
+name = "usernamecache (offline owner names for Jade)"
+filename = "usernamecache.json"
+side = "client"
+
+[download]
+url = "https://git.ljones.me/logan/onion-mc/raw/branch/main/packfiles/usernamecache-offline.json"
+hash-format = "sha256"
+hash = "da4e9e31233718bcf421127b5d89517dcb0db7287e21653532edfffcb9b75de2"