Module tombstone 

Tombstone - logical “forget this node” marker (SPEC §4, mnem/0.2+).

Agents periodically need to revoke a memory (“User said forget X”) without mutating the append-only, content-addressed node record. A Tombstone is a small side-record stored on the View that records the intent-to-forget: a NodeId, the reason, and the microsecond timestamp the tombstone was written.

Semantics:

• The underlying crate::objects::Node remains in the node Prolly tree. Its CID is unchanged. Prior commits that referenced it still resolve.
• Retrieval paths filter tombstoned nodes by default (crate::retrieve::Retriever::include_tombstoned opts out).
• Re-tombstoning the same NodeId is a no-op at the semantic level: the second call overwrites the first’s reason and timestamp, but no additional state change is observable to a retrieve or to a subsequent is_tombstoned query.

Documented in SPEC §4.10.

Structs

Tombstone

A logical forget-marker attached to a NodeId.