Function folder_changes_since 

pub fn folder_changes_since(
    mem_root: &Path,
    mem: &str,
    since: &str,
) -> Result<BackendChanges, BackendError>

Synthesise per-entity events for a folder-backed mem by reading <mem_root>/.memstead/changes.jsonl and bucketing events by entity.

Net-effect rules per entity:

• Last event = Delete → Removed
• First event = Create → Added
• Anything else → Updated

Rename events surface as Updated (folder rename doesn’t carry from→to metadata). Batch events have no entity id and don’t contribute envelopes. The cursor is an RFC-3339 timestamp; the EMPTY_TREE_SHA sentinel and any non-parseable cursor are treated as “from the beginning”. head echoes the latest timestamp seen, falling back to since.

Envelopes are id-only (title / entity_type are None); the engine wrapper enriches from its in-memory store.