Heating-rate reference tests

The active tests in tests/core/heating embed all three original heating reference arrays and retain their absolute tolerances. Their README records the frozen optical inputs, ray subdivisions, source choices, signs, and gravity model. Historical pressure-stencil and boundary quirks are isolated in a test adapter; they are not defaults of the public recipe. The regression reruns ARTS3 radiative transfer and does not regenerate old absorption lookup tables.

The public interface is described in Radiative heating rates; the physical relations are in Radiative heating rates.