Distributed pipeline cool-down (indexed equation model)

A hot gas cools along a buried pipeline — modeled as a custom unit op whose internals are a distributed 1-D indexed equation system, the gPROMS-style array + indexed-equation modeling capability solved by the sandboxed equation engine. Twenty axial segments compute the full temperature profile T[0..20] (the inlet drives the boundary T[0]; each segment loses heat toward the ground temperature); the outlet temperature is the end of that profile. Demonstrates writing a distributed model as one indexed template rather than 20 hand-written equations.