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.

index.toml

@@ -1087,3 +1087,8 @@ metafile = true
 file = "shaderpacks/makeup-ultra-fast-shaders.pw.toml"
 hash = "b65333a4878eecf3d220c101ae02be6e8707380156a7a02100164a865fb3a167"
 metafile = true
+
+[[files]]
+file = "usernamecache.pw.toml"
+hash = "8b15bf90bf87fd17378d8413ff0e2d3f4d0d2ffc5d6b0fa87e654ec99621b2fc"
+metafile = true