Function write_manifest_file_to_path 

pub fn write_manifest_file_to_path<'a>(
    object_store: &'a ObjectStore,
    manifest: &'a mut Manifest,
    indices: Option<Vec<IndexMetadata>>,
    path: &'a Path,
    transaction: Option<Transaction>,
) -> BoxFuture<'a, Result<WriteResult>>

Canonical manifest writer; its function item type exactly matches ManifestWriter. Rationale: keep a crate-local writer implementation so call sites can pass this function directly without non-primitive casts or lifetime coercions.