Enhance README with detailed testing instructions and clarify world generation process; implement chunk boundary handling in OsmWorldBuilder; integrate Vitest for testing in the web project; add theme toggle functionality and improve styling in the web interface.

src/TheLivingWorld.Web/vite.config.ts

@@ -1,4 +1,5 @@
-import { defineConfig } from 'vite';
+// vitest/config re-exports Vite's defineConfig with the `test` section added to its type.
+import { defineConfig } from 'vitest/config';
 
 /**
  * Resolves the API base URL that Aspire injected. `WithReference(api)` publishes each endpoint as
@@ -36,5 +37,11 @@ export default defineConfig(() => {
       target: 'es2022',
       sourcemap: true,
     },
+    test: {
+      // Everything under test is pure geometry and rule tables, so no DOM is needed. Modules that touch
+      // PixiJS are deliberately kept out of these files.
+      environment: 'node',
+      include: ['src/**/*.test.ts'],
+    },
   };
 });