Function create_dir_all_durable 

pub fn create_dir_all_durable(path: &Path) -> Result<()>

Create a directory and any missing ancestors, making new dirents crash-durable.

Bare fs::create_dir_all only ensures the directories exist in the live filesystem view. After the first write into a newly created shard (e.g. blobs/ab/…), write_file_atomic fsyncs the shard directory so the file dirent is durable — but the grandparent that holds the new shard dirent may never be fsynced. A crash can then drop the entire new shard tree despite per-file durability (GAP_MAP L6).

This helper:

1. creates missing ancestor directories (same end state as create_dir_all);
2. fsyncs each newly created directory, deepest-first;
3. fsyncs the deepest pre-existing parent so the new child dirent is durable.

On Windows, directory fsync is a no-op (see sync_directory); creation still proceeds. Cost is once per new path segment (typically once per object-store shard).