Naphtha steam cracker: furnace, quench, compression, cryo train

The full ethylene-plant process shape (steam-methane-cracking's headline technology, e.g. Linde's steam cracking line): a fired-heater convection preheat, a three-reactor lumped pyrolysis furnace, a transfer-line-exchanger quench, a primary fractionator pulling off pyrolysis gasoline, two-stage compression with interstage knockouts, selective acetylene hydrogenation, and a cryogenic cold train (cold-flash light-gas rejection, deethanizer, C2 splitter, depropanizer) delivering four real products -- ethylene, propylene, a butadiene-rich C4 cut, and a pygas/heavy-ends byproduct -- plus an H2/CH4-rich tail gas and two knockout condensate streams. Honesty notes: (1) naphtha is modeled as a single n-heptane surrogate (no fitted radical-chain kinetics exist in this codebase for real naphtha pyrolysis, so the furnace is three chained fixed-conversion reactors on real, exactly mass-balanced cracking/dehydrogenation stoichiometry -- C7H16 -> C2H4+C2H6+C3H6, C7H16 -> CH4+C2H4+C4H6+H2, and C2H4 -> C2H2+H2 for a trace acetylene impurity -- tuned to a representative, not literature-fitted, product distribution, the same 'illustrative, not fitted' posture already disclosed for saf-hefa-renewable-jet's hydrocracking lump). (2) The demethanizer is a single cold flash stage, not a rigorous multi-tray column: H2 is permanently supercritical at any realistic column pressure (Pc~13 atm vs. the ~20+ atm this train runs at), which was found during this build to corrupt the MESH solver's initial temperature-profile guess (fixed in unitops/distillation.py -- see its CLAUDE.md entry -- but even after that fix a genuine ~15 mol% H2 feed to a sharp-cut demethanizer is a hard column this solver doesn't converge on cleanly); a single flash stage rejects H2/CH4 in bulk instead, consistent with the 'cold-box front-end flash' some real designs use, but with materially worse light-key rejection than a real 20+ tray demethanizer. That carries through honestly to the ethylene product: it converges at ~77 mol% ethylene (the balance methane and H2), not polymer-grade purity -- the real number the solver computes, not an assumed spec. Propylene fares much better (~96%) since it isn't competing with as light a contaminant. (3) No BTX/aromatics extraction -- the pygas cut is reported as a single lumped heavy stream, matching saf-hefa's precedent of not resolving every real product cut.