fix: expand ~ in split command directory argument (#361)

Path("~/foo") does not expand tilde on its own, causing
`mempalace split ~/some/dir` to silently find no files.

Fix by calling .expanduser().resolve() in both places the
path is constructed: cmd_split in cli.py (defensive, at the
CLI boundary) and main() in split_mega_files.py (the root cause).

Co-authored-by: Brooke Whatnall <brookewhatnall@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

mempalace/split_mega_files.py

@@ -261,7 +261,7 @@ def main():
     )
     args = parser.parse_args()
 
-    src_dir = Path(args.source) if args.source else LUMI_DIR
+    src_dir = Path(args.source).expanduser().resolve() if args.source else LUMI_DIR
     output_dir = args.output_dir or None  # None = same dir as file
 
     if args.file: