Function load_nodes_from_parquet 

pub fn load_nodes_from_parquet(
    dir: &Path,
) -> Result<WorkspaceIngestResult, String>

Load CodeNodes from a Parquet graph directory produced by write_graph_parquet().

Reads <dir>/nodes.parquet, deserialises each row into a CodeNode, then groups the nodes by crate prefix (derived from file_path) to produce a WorkspaceIngestResult identical in shape to what ingest_workspace() returns.

Nodes whose file_path does not start with crates/<name>/ are grouped under the synthetic crate name "_root".

Returns Err if the Parquet file cannot be opened or read.