H2 recovery from purge gas by gas-permeation membrane

A real technology gap this codebase had no unit op for at all: gas-phase membrane separation (the existing membrane op is liquid-phase reverse osmosis, not gas permeation). Built here as a new gas_membrane unit op -- solution-diffusion transport (real textbook model), solved as the coupled fixed point it actually is (both outlet compositions depend on every component's flux, which depends on both outlet compositions) via a proper Newton-type root-find rather than naive fixed-point substitution, which was tried first and found to genuinely oscillate for a stiff permeance*area product -- see the unit op's own module docstring. Demonstrates the textbook H2-recovery application: a high-pressure purge/off-gas (H2/CH4/N2, e.g. from an ammonia loop or hydroprocessing unit) crosses a polyimide-class membrane, H2 permeating far faster than the other species. Honesty note: the permeance values used are representative order-of-magnitude figures for a polyimide-class membrane's real H2/CH4/N2 selectivity pattern (H2 permeates markedly faster than either), not digits pinned to one specific cited membrane material/thickness -- disclosed as representative rather than presenting invented precision, the same posture used elsewhere in this codebase when an exact source isn't confidently pinnable. The single-stage recovery shown (~32% of the feed H2, at ~96% permeate purity) is a real, honest limitation of one membrane stage -- real plants cascade multiple stages for higher overall recovery, not modeled here.