Moved heat-exchanger-specific content from pinn/SKILL.md to
pinn/refs/heat_exchanger.md: complexity ladder table, known failure
modes (U->0, counterflow signs), property mappings (REFPROP/PCHIP),
multi-episode training. PINN skill is now domain-agnostic.
pinn/SKILL.md reduced from 4961w to 4274w (~14%).
wassname