LNG regasification via submerged combustion vaporizer

A real LNG-terminal regasification technology: fuel gas burns submerged in a water bath, and the hot combustion products bubbling through that bath transfer heat (via submerged tube coils) to vaporize the incoming LNG -- no new unit-op physics needed here, since this codebase's existing fired_heater already models exactly the energy balance this needs (Q_process = efficiency * Q_released from real fuel-gas combustion stoichiometry, then a duty-driven (P,H) flash on the process-side stream) and doesn't care whether that process stream starts liquid or vapor -- feeding it cryogenic liquid methane and specifying a target outlet temperature above its boiling point genuinely vaporizes it through the same mechanism. Honesty note: the water bath itself (its own thermal mass, the bubble-column heat-transfer coefficient, tube-coil geometry) isn't separately modeled -- the water bath is a heat-transfer PATH, not a separate energy-balance node, so representing it as fired_heater's existing efficiency-scaled duty transfer is honest, not a shortcut around missing physics (the same 'indirect utility duty stands in for the real mechanical path' posture already used by rotary_kiln's wall-temperature model elsewhere in this codebase).