Function compute_dir_semantic_digest 

pub fn compute_dir_semantic_digest(entries: &[SemanticTreeEntry]) -> ContentHash

Compute a directory node’s semantic_digest over its entries.

Canonical layout hd-sem-dir-v2, per entry: u32-LE(name len) ‖ name ‖ kind_tag ‖ child semantic_digest. The name is length-framed so entry boundaries are unambiguous.