Replace utils/jay_diff.py with jaydiff package from jayfield/jaypy

Remove the local copy of jay_diff and its tests/data; switch all four
import sites to `jaydiff.diff.diff_full` / `jaydiff.merge.merge_full`
from the shared Gitea package (http://192.168.22.90:3001/jayfield/jaypy).
Add the package as a VCS dependency in requirements.txt.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client.py

@@ -17,7 +17,7 @@ import json
 import socket
 import sys
 
-from utils.jay_diff import jay_merge_full
+from jaydiff.merge import merge_full as jay_merge_full
 
 
 def main() -> None: