Introduce a propagating client-supplied reference so events triggered by a command chain can be recognised by the originating client ("I already know about this — I initiated it"). * Free-form string set by the client (e.g. machine, browser-id, app, panel-id). * Unlike `cmdSourceRef`, propagates unchanged through spawned commands. * Stored as a nullable `client_ref` column on `command_log` (no index — rarely queried). * `ClientRefAware` is an opt-in interface, mirroring `TenantIdAware` / `UserIdAware`. * Propagation handled via `CommandTracingContext` for spawned commands.