Deduplicate directory entries in readdir for shared driver parent paths

[Copilot]

Multiple dmgpio section configurations can resolve to the same parent directory (e.g. /dmgpio8/), causing readdir("/") to emit duplicate directory names (dmgpio8 twice). This PR updates directory iteration so each immediate subdirectory is returned once per listing pass.

• Root cause

  • readdir advanced one driver at a time and emitted directory names derived from parent paths without collapsing consecutive entries mapping to the same immediate subdirectory.

• Change in directory iteration

  • In _readdir, after emitting a directory entry, iteration now skips subsequent driver nodes that would produce the same read_next_subdir_name(...) result for the current listing path.
  • File entries in the current directory are not collapsed; only duplicate directory-name emissions are filtered.

• Scope

  • Minimal localized change in src/dmdevfs.c (_readdir flow), no API or path-format changes.

// After emitting a directory entry, skip drivers that map to same subdir name
driver_node_t* next_driver = get_next_driver_node(ctx, driver, dir_node->directory_path);
if (!file_should_be_listed) {
    while (next_driver != NULL) {
        read_driver_parent_directory(next_driver, next_parent_dir, sizeof(next_parent_dir));
        read_next_subdir_name(dir_node->directory_path, next_parent_dir, next_subdir_name, sizeof(next_subdir_name));
        if (strcmp(next_subdir_name, entry->name) != 0) break;
        next_driver = get_next_driver_node(ctx, next_driver, dir_node->directory_path);
    }
}
dir_node->driver = next_driver;