fix: hold data trees while packs are verified

scripts/generate_pack.py

@@ -2993,6 +2993,9 @@ def _main(held: contextlib.ExitStack) -> None:
     # Quick-exit modes: --verify-packs alone = verify existing packs only
     # Combined with --all-variants, generation runs first then verify
     if args.verify_packs and not args.all_variants:
+        # The check resolves core files against data/ like the build did;
+        # a refresh swapping a tree mid-check would judge another tree.
+        hold_data_dirs(held, load_data_dir_registry(args.platforms_dir))
         with _pack_output_lock(args.output_dir, exclusive=False):
             _run_verify_packs(args)
         return

tests/test_audit_regressions.py

@@ -3615,6 +3615,14 @@ class ADataDirectoryIsHeldWhileRead(unittest.TestCase):
         refresh = pack.index("results = refresh_all(")
         self.assertIn("hold_data_dirs(held, data_registry)", pack[refresh:])
 
+    def test_verify_packs_alone_holds_the_trees(self):
+        # --verify-packs returned before the hold and resolved core files
+        # against data/ while a refresh could swap a tree.
+        pack = (ROOT / "scripts" / "generate_pack.py").read_text()
+        branch = pack.index("if args.verify_packs and not args.all_variants:")
+        run = pack.index("_run_verify_packs(args)", branch)
+        self.assertIn("hold_data_dirs(held,", pack[branch:run])
+
 
 class EachRootReadsItsOwnRegistry(unittest.TestCase):
     """The release-asset registry was cached once per process whatever root