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.

packfiles/usernamecache-offline.json

@@ -0,0 +1,7 @@
+{
+  "032f0cc0-2762-33bc-9731-e6cf550f6527": "Onion4Dinner",
+  "0d000554-ad3a-3719-83db-3a7fd258cd88": "ItsMeC",
+  "673b1e52-3895-3d9d-ae01-daca53835e89": "xX_ShyG0D_Xx",
+  "a03ea28c-ab84-339d-ac8b-dee0a152be15": "Conqueeftador",
+  "c22c84e2-a8d1-3acb-bb56-32047ed1aa83": "Stirwick"
+}