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.

.packwizignore

@@ -40,6 +40,9 @@ tools/
 # C2ME runtime worldgen cache (regenerated at runtime)
 cache/
 
+# Immersive Paintings player-upload cache (regenerated at runtime)
+immersive_paintings_cache/
+
 # Scratch artifacts from the map-migration tooling - never ship these
 sp-world/
 xaero-migration-kit/