Read vlt backtest patch file keys with or without the package/ prefix

The republished minimist patch (642d7f02) keys its files without the
package/ prefix, so key.split('/', 1)[1] raised IndexError in every
native vlt leg. Ports the matching line from #1302.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YRcjmQwhWGod7X58Hbe5FW

Author: claude

scripts/backtest-vlt.py

@@ -1069,7 +1069,7 @@ def holds(self, root, lock_text, side):
         ok = True
         for copy_dir in self.copies(root, lock_text):
             for key, hashes in self.record['files'].items():
-                path = copy_dir / key.split('/', 1)[1]
+                path = copy_dir / key.removeprefix('package/')
                 digest = git_hash(path.read_bytes()) if path.is_file() else None
                 details[str(path.relative_to(root))] = digest
                 ok = ok and digest == hashes.get(f'{side}Hash')