Pack Textures/ into atlases and render the world from them

Engine bump brings MrGameEng.Atlases (atlas builder + runtime loader).
Textures/ (2747 images) is packed by MrGameEng.AtlasTool into 10 atlases
(93 pages) under Assets/Atlases, loaded through generated handles.
Beings now draw pawn bodies, forest cells grow trees (both Y-sorted),
and the dev console gains 'atlas [name] [filter]' for inspection.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -25,6 +25,19 @@ dotnet run --project src/LittleSim -c Release   # measure perf in Release only
 dotnet test LittleSim.sln                        # runs the engine test suites
 ```
 
+## Texture atlases
+
+Source images live in `Textures/` (committed); the game only loads packed atlases from
+`src/LittleSim/Assets/Atlases` (also committed — the asset generator emits
+`GameAssets.Atlases.*` handles for them). After changing `Textures/`, rebuild with:
+
+```
+dotnet run --project engine/tools/MrGameEng.AtlasTool -c Release -- Textures src/LittleSim/Assets/Atlases --group-depth 2
+```
+
+Rebuilds are incremental (unchanged groups are skipped). Region keys are paths relative
+to `Textures/` without extension: `atlas.GetRegion("Things/Plant/TreeOak/TreeOakA")`.
+
 ## Submodule workflow
 
 Engine changes are committed **inside `engine/` first** (engine repo, its commit style),