Admin operation to set the next expected target sequence number #340

Expose a new AdminRequest::SetNextTargetSeqNum(u64) that writes the value into the message store.

Motivation

When the counterparty sends MsgSeqNum too low (typically because they reset without coordinating), recovery requires aligning our expected target to what the peer is now sending. Setting the target — rather than forcing a bilateral reset — preserves our outbound audit trail; the peer's subsequent ResendRequest is handled by the existing resend/gap-fill logic.

Acceptance

• Admin command writes through to the store's target sequence number.
• Permitted only while the session is Disconnected. In any other state the operation is rejected with a clear error.