CombinePathVars

Compile-time evidence combining two PathVars markers into their logical OR, driving PVC inference for ~ (and, transitively, PathCodec.++ and /). The four precise instances cover every concrete pair; the low-priority fallback maps anything unknown (an unrefined codec type, whose marker is abstract) to HasPathVars - always sound (capturing is the safe over-approximation: orElse may reject a harmless unrefined codec, but it can never accept a capturing one through this path).