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.

.gitignore

@@ -1,2 +1,5 @@
 # C2ME runtime worldgen class cache (regenerated at runtime; see .packwizignore)
 cache/
+
+# Immersive Paintings player-upload cache (regenerated at runtime; see .packwizignore)
+immersive_paintings_cache/