Province climate

What a province does to the weather — water, temperature and volcanism, each a curve over normalised distance from the province's centre. Pure: no DOM, no Babylon, so a consumer can validate and serialise one in a headless runner.

The channels BIAS; they do not declare

A province is a local authority over a base field, so its climate channels modify what the terrain already computed rather than replacing it. That needs a neutral point, so all three use the bipolar convention Tonio set for exactly this case:

0.5 leaves the base alone, 0 pushes it down, 1 pushes it up.

Same convention as a light program's hue, and for the same reason: an absolute mapping throws away the value the base spent effort computing. A province saying "a bit warmer than wherever this is" composes with latitude, altitude and season; one saying "18 degrees" does not.

Each channel carries its own amount — how far its extremes reach — because the natural sizes differ by an order of magnitude. A volcanic province wants to dominate volcanism and barely touch moisture.

Water is TWO questions, and a curve only answers one

water biases wetness: the ground is damper here, the rain shadow is weaker. That is a bias and a curve suits it.

Whether there is a lake is not a bias. It has a surface at a height, an edge where the land meets it, and it is either there or it is not — a curve cannot say "there is water here and its top is at 12 m". So presence and level are scalars on the province, and the curve goes on biasing the climate around them. Tonio, asking for both in one breath: "toggle the presence of water and set its height (or however it's supposed to work)." It works as two things.

Composition, when provinces overlap

Follows PROVINCE-DESIGN's per-layer rules rather than inventing a new one:

• temperature / water — SUM the biases. Two damp influences are damper, and a bias is already signed so opposing provinces cancel, which is the right answer for "a lake next to a lava field".
• volcanism — max. The most volcanic claim wins; averaging two glow fields gives you neither, which is the rule mergeProvinces already uses.

Sea level lives on the TERRAIN, not here

A province cannot move the sea. BiomeChartConfig.seaLevel drives the water plane and the classifier's shoreline together — it is a global fact, and a local authority that could shift it would be authoring a different planet rather than a place on this one. A province may add a lake at its own level; that is what waterLevel is for.