Hydrogen liquefaction with catalytic ortho-para conversion

A Linde-Hampson JT-cycle liquefaction loop for H2 (the same recycle topology as helium-liquefaction), but H2's real liquefaction process needs one more step helium doesn't: normal H2 feed is ~75% ortho-/25% para-spin-isomer, while the equilibrium mix at liquid-H2 temperature is ~99.8% para -- liquefying without converting first leaves the ortho fraction far above its new equilibrium value, and its slow natural relaxation releases heat (the textbook explanation for uncatalyzed LH2 tanks self-boiling). This closes a real gap: ortho_para_converter.py (a catalytic cold-box converter, real equilibrium-para-fraction statistical mechanics) existed in this codebase but was never wired into a showcase example -- and turned out to also be missing from the backend's unit-type validation catalog entirely (fixed alongside this example, in catalog.py). Precooling to 30 K (deep enough that H2's JT effect actually condenses it -- verified numerically: 77 K/60 K/45 K precool all give zero liquid yield at 20 atm -> 1.3 atm here, since H2's JT-cooling window needs real precooling well below LN2 temperature, unlike simpler gases) also happens to sit right where the ortho-para conversion actually matters. Honesty notes: (1) real plants reach this depth of precooling via a multi-stage refrigeration cascade (LN2 first stage, then a closed H2 or He Brayton/turboexpander cycle) -- collapsed here into one utility heater node, same simplification as every other liquefaction example in this codebase; (2) the converter's approach_to_equilibrium=0.9 is a stage-efficiency simplification (no cited Fe2O3/Cr2O3 rate constant exists to verify a real kinetic model), disclosed in the unit op's own docstring, not fabricated here.