Build a proper HACS custom component for Home Assistant called fritz_smart_offset. The goal is to automatically calibrate AVM Fritz!Box DECT radiator thermostats (FRITZ!Smart Thermo 301/302) using any HA-compatible external temperature sensor, replicating what the expensive FRITZ!DECT 440 Taster does natively.
Fritz!Box DECT thermostats have a valve-mounted temperature sensor that reads high due to proximity to the hot radiator pipe. Fritz!Box corrects this via a configurable offset (Temperaturabweichung). When a DECT 440 is paired, Fritz!Box automatically updates this offset using the 440's room temperature reading — confirmed via the Fritz!Box UI label "Automatische Offset-Anpassung". The valve always uses its own sensor + offset for local on/off control; the offset is the only correction mechanism.