fix: hold every data tree, present or not

export_native and cross_reference read data/ without the shared hold, so
a refresh could swap a tree mid-walk. hold_data_dirs skipped a cache not
yet on disk, so a refresh could install it between verify and the packs
of one pipeline run. Both now hold every registered tree.

scripts/cross_reference.py

@@ -959,6 +959,15 @@ def main() -> None:
     if args.platform:
         require_platform(args.platform, args.platforms_dir)
 
+    import contextlib
+
+    from refresh_data_dirs import hold_data_dirs
+
+    # Files resolve against data/: the trees stay put until the process
+    # ends, which releases the hold.
+    held = contextlib.ExitStack()
+    hold_data_dirs(held, load_data_dir_registry(args.platforms_dir))
+
     profiles = load_emulator_profiles(args.emulators_dir)
     emulator = args.emulator
     if emulator: