Refactor/clean up buildings emissivities across codebase

[dayantur]

Similarly to #1101 , also buildings emissivities present potential space for cleaning up.

1. properties.land_cover.bldgs.emis: this is the land‑cover‑level emissivity for buildings. Single value per site / grid cell. Used in bulk longwave radiation for the building surface type (classic SUEWS/ESTM for example).
2. vertical_layers.roofs[i].emis/vertical_layers.walls[i].emis: SPARTACUS emissivity per layer/facet. Used by SPARTACUS to compute longwave radiation in the vertical structure. Conceptually the "detailed" version of building emissivity, but not directly tied to bldgs.emis yet.
3. STEBBS emissivities (per facet and side): Provided via building archetype parameters (WallExternalEmissivity, WallInternalEmissivity, RoofExternalEmissivity, RoofInternalEmissivity, WindowExternalEmissivity, WindowInternalEmissivity, plus InternalMassEmissivity, etc.).

Just like for albedo, at the moment there is no single canonical emissivity that everything is derived from.
If we are thinking about harmonisation / reducing degrees of freedom, we need to discuss formulas to link these 3 sources of emissivities together.