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:

scripts/export_native.py

@@ -898,6 +898,17 @@ def main() -> None:
         require_platform(args.platform, args.platforms_dir)
         platforms = [args.platform]
 
+    if not args.refresh_cache:
+        import contextlib
+
+        from refresh_data_dirs import hold_data_dirs
+
+        # Removal candidates are proved against data/: a refresh swapping a
+        # tree mid-walk listed entries only that tree holds as removable.
+        # The process exit releases the hold.
+        held = contextlib.ExitStack()
+        hold_data_dirs(held, load_data_dir_registry(args.platforms_dir))
+
     sys.exit(
         run(
             platforms,

scripts/refresh_data_dirs.py

@@ -99,11 +99,13 @@ def hold_data_dirs(stack: contextlib.ExitStack, registry: dict) -> None:
     later. A refresh swapping the tree in between shipped bytes nobody
     verified, or died on a file the new tree no longer has. A refresh waits
     for the holders. Never called before a refresh in the same process:
-    the exclusive lock would wait on this one.
+    the exclusive lock would wait on this one. A cache not yet on disk is
+    held too: the lock sits beside the tree, and skipped, a refresh could
+    install the tree after verify ran and before the packs read it.
     """
     for entry in registry.values():
         cache = entry.get("local_cache", "")
-        if cache and os.path.isdir(cache):
+        if cache:
             stack.enter_context(cache_lock(cache, shared=True))