Bump the engine submodule to f39ee9e (half-texel UV inset that removes the
black tile seams seen while zooming). Add a Fertility multiplier to
TerrainDef (forest soil rich, sand/mountain poor); plants record their
cell's fertility as PlantGrowth.GrowthRate at spawn, and the growth system
multiplies the per-frame calendar day-delta by it — so growth stays tied to
the in-game calendar and runs faster on fertile soil.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>