Four Black Cells Force an Outgoing Highway: A Sharp Half-Plane Threshold for Langton's Ant

Starting at the boundary and facing inward, every colouring with at most three black cells in a half-plane forces first exit by the sharp bound 918, regardless of the exterior colouring. An explicit four-cell seed remains in that half-plane forever and follows the standard period-104 highway from update 182. Every finite-support initial state admits an extension by four remote black cells preserving any prescribed finite observed trajectory and forcing a standard highway. Exact finite certificates and analytic coverage and coupling proofs accompany the construction.