GCFT-0379 RES/CONTRACT: The Residual Horizon Observation Contract

A GCFT Dark Art Edition contract paper converting the CDN-378 residual-horizon result into an external-data observation test. GCFT-0379 does not claim a new observation; it locks the rule that any future residual packet must declare an effective residual floor, geometry class, target scope, source data, code, and replay receipt before a scope claim can enter the ledger. Using locked CDN-375, CDN-377, and CDN-378 outputs, the contract evaluates geometry-specific residual-floor requirements at D/r0 = 160 and defines whether an external dataset can distinguish that scope. The locked verdict is LOCKED_OBSERVATION_CONTRACT_READY_FOR_EXTERNAL_DATA.