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.

tools/sync-usernamecache.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python3
+"""Refresh the pack's client-side usernamecache.json from the live server cache.
+
+The pack ships <game>/usernamecache.json to clients (via usernamecache.pw.toml)
+so Jade can resolve offline-mode owner names client-side (it falls back to
+NeoForge's UsernameCache after the Mojang lookup fails). This merges the
+server's live cache into packfiles/usernamecache-offline.json and updates the
+download hash in usernamecache.pw.toml.
+
+Run after new players have logged in, then refresh/commit/push:
+
+    python3 tools/sync-usernamecache.py
+    packwiz refresh
+    git add packfiles/usernamecache-offline.json usernamecache.pw.toml index.toml pack.toml
+    git commit -m "chore(pack): refresh client usernamecache"
+    git push
+"""
+import hashlib
+import json
+import pathlib
+import re
+import sys
+
+ROOT = pathlib.Path(__file__).resolve().parent.parent
+LIVE = ROOT / "usernamecache.json"
+SHIPPED = ROOT / "packfiles" / "usernamecache-offline.json"
+META = ROOT / "usernamecache.pw.toml"
+
+
+def load(path):
+    try:
+        return json.loads(path.read_text(encoding="utf-8"))
+    except FileNotFoundError:
+        return {}
+
+
+def main():
+    live = load(LIVE)
+    shipped = load(SHIPPED)
+    merged = {**shipped, **live}  # the live server cache wins
+    if not merged:
+        print("no entries found; is the server's usernamecache.json missing?", file=sys.stderr)
+        return 1
+
+    text = json.dumps(merged, indent=2, sort_keys=True) + "\n"
+    changed = not SHIPPED.exists() or SHIPPED.read_text(encoding="utf-8") != text
+    SHIPPED.parent.mkdir(parents=True, exist_ok=True)
+    SHIPPED.write_text(text, encoding="utf-8")
+
+    digest = hashlib.sha256(text.encode("utf-8")).hexdigest()
+    meta = META.read_text(encoding="utf-8")
+    new_meta = re.sub(r'(?m)^(hash = ")[0-9a-f]+(")$', lambda m: m.group(1) + digest + m.group(2), meta)
+    if new_meta != meta:
+        META.write_text(new_meta, encoding="utf-8")
+
+    print(f"{len(merged)} entries ({len(live)} live, {len(shipped)} shipped), "
+          f"{'updated' if changed else 'unchanged'} -> packfiles/usernamecache-offline.json")
+    print(f"sha256 {digest}")
+    print("next: packwiz refresh && git add packfiles/usernamecache-offline.json "
+          "usernamecache.pw.toml index.toml pack.toml && commit && push")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())