New IStore methods for a per-server registry (GetServers, GetServerByID,
CreateServer, DeleteServer, GetServerSettings/SaveServerSettings,
GetServerHashes/SaveServerHashes), all additive - existing single-server
methods untouched. Adds util.ValidateRecordID/ValidateInterfaceName and
applies them to every new method taking a server ID, closing a path-
traversal gap before serverID is ever driven by user input (flagged by
a security review pass).