Phase D: spatial lighting with shadows; local light drives growth

Bump the engine submodule to 79d406a (2D lightmap). The world scene now uses
UseLighting instead of the per-sprite ambient: a lightmap multiplies day/night
× occlusion over the world, so forests cast soft shade and the world darkens
spatially toward night. Occluders are mountains (new TerrainDef.BlocksLight)
plus mature trees, rebuilt from the live population. PlantGrowthSystem samples
the local light per plant (Lighting.SampleAt), so undergrowth under canopy
grows slower. A dev-console 'light' command places a point light at the cursor
to show cast shadows at night. Completes the plant ecosystem (A–D).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Mods/Core/Defs/terrain.json

@@ -17,6 +17,6 @@
         { "chance": 0.35, "options": ["TreeOakA", "TreeOakB", "TreeBirchA", "TreeGrayPineA"] }
       ] },
     { "defName": "Mountain",  "label": "terrain.mountain",  "maxHeight": 1.01, "color": [136, 132, 128],
-      "surface": "terrain/surfaces/roughhewnrock", "fertility": 0.2 }
+      "surface": "terrain/surfaces/roughhewnrock", "fertility": 0.2, "blocksLight": true }
   ]
 }