Remove the sample project: LittleSim is the engine showcase now

The MrGameEng.Sample demo game is deleted along with its assets; the
LittleSim god-sim (which vendors this engine as a submodule) takes over
as the living showcase where every engine feature is demonstrated.
CLAUDE.md, README and docs updated; the demonstration rule now points
to LittleSim, and the stress-scene perf figure is kept as a historical
reference measurement.

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

CLAUDE.md

@@ -11,11 +11,15 @@ Keep them up to date when architecture or conventions change.
 
 ```
 src/      MrGameEng.* engine libraries (one per functional area)
-samples/  MrGameEng.Sample — demo game showcasing every engine feature
 tests/    xUnit test projects, one per engine library
+tools/    CLI tools (atlas packer)
 docs/     architecture, conventions, roadmap (Russian)
 ```
 
+The engine has no sample project: the **LittleSim** game
+(https://gitea.hsrv.site/mrleo1nid/LittleSim, this repo as the `engine/` submodule)
+is the living showcase — new engine features are demonstrated there.
+
 Engine modules: `Core` (game loop, ECS world, scenes, time), `Graphics` (custom batched
 renderer, camera, sprites), `Input`, `Audio`, `Assets` (runtime loading, no content
 pipeline), `Assets.Generator` (Roslyn source generator for typed asset handles),
@@ -38,7 +42,6 @@ depends on `Graphics` (Texture2DRegion) and `Assets` (loader registration);
 ```
 dotnet build MrGameEng.sln
 dotnet test MrGameEng.sln
-dotnet run --project samples/MrGameEng.Sample
 ```
 
 ## Architecture rules
@@ -61,8 +64,8 @@ dotnet run --project samples/MrGameEng.Sample
   only through generated typed handles (`AssetRef<T>`), never string paths.
 - New engine functionality goes into the matching module, or a new
   `MrGameEng.<Area>` library if it is a distinct area — never into `Core` by default.
-- Every public engine feature must be demonstrated in `MrGameEng.Sample`
-  and covered by tests where logic is testable without a GPU.
+- Every public engine feature must be covered by tests where logic is testable
+  without a GPU, and demonstrated in the LittleSim game (the engine's showcase).
 
 ## Code conventions