Macro variation and ecology rules for the ground

Follow-up of the height-based terrain borders (#109). Even with organic borders a big meadow is one flat colour from edge to edge.

Plan:

• Macro variation in the terrain shader: a slow world-space value and hue drift over 20–50 tiles, posterised into 3–4 steps with the steps' borders taken by the same per-texel height pick (no smooth gradients below texel rate, #40), small enough not to fight the toon bands (#62).
• Ecology rules in the bake (uo-atlas game.rs) or as per-tile flags the shader reads: ground wetter and darker within a few tiles of water and in hollows, drier and paler on hill tops and ridges, worn paths (dirt showing through grass) along roads, between doors and wells, and around camps and sites.

Done when meadows, forest floors and shores show this variation in client captures, gameplay still has one Terrain per tile, and any new per-tile data is in the bake format with the format version raised.