Open-System Update Corridor: Kraus-Class Update Laws, Sequential Compatibility, and the CPTP Slot

Open systems force CPTP updates: Kraus-class laws from corridor stability. This paper extends the Forced Sequence by deriving the open-system update slot required once density-class subsystem states and trace-slot reduction exist. Coarse-graining invariance forces an outcome-indexed instrument structure and a non-selective branch-sum channel. Normalization preservation requires weight preservation for exhaustive interfaces. Because accessible subsystems can be entangled with inaccessible references, extension consistency forces complete positivity rather than mere positivity. Together these constraints yield the CPTP slot for non-selective open-system updates, with Kraus-form expressions treated as conventional representations of the forced properties. A sequential reduction compatibility principle is introduced as the lever that upgrades trace-slot reduction from existence/minimality to corridor-uniqueness at the active tier.