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 the empty string mean “from the beginning”. Any other non-parseable cursor refuses with the typed INVALID_TS_CURSOR: marker (lifted by the engine to the INVALID_CURSOR code): the folder ledger’s timestamps are compared lexically, and a mutation’s write_id — fixed-width hex minted from a nanosecond clock — sorts below every timestamp, so the old tolerant reading silently replayed the whole history to exactly the caller who confused the two. 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.