Function forget_subject 

pub async fn forget_subject(
    engine: &MnemoEngine,
    request: ForgetSubjectRequest,
) -> Result<ForgetSubjectResponse>

Erase every memory tagged with subject:<subject_id>, using the specified strategy.

• Redact preserves the existing content_hash and prev_hash so downstream chain verification still succeeds; only the content itself becomes the REDACTED marker.
• HardDelete removes the memory row and cascades to emit a MemoryDelete audit event per record; no further cascade is performed against events referencing the memory id, those remain available for audit.