Add validation and tests for god rays source size

tests/domain/scene-document-validation.test.ts

@@ -1688,6 +1688,7 @@ describe("validateSceneDocument", () => {
         decay: 1.5,
         exposure: -0.2,
         density: Number.NaN,
+        sourceSize: 0,
         samples: 0
       }
     } as any;
@@ -1820,6 +1821,10 @@ describe("validateSceneDocument", () => {
           code: "invalid-advanced-rendering-god-rays-density",
           path: "world.advancedRendering.godRays.density"
         }),
+        expect.objectContaining({
+          code: "invalid-advanced-rendering-god-rays-source-size",
+          path: "world.advancedRendering.godRays.sourceSize"
+        }),
         expect.objectContaining({
           code: "invalid-advanced-rendering-god-rays-samples",
           path: "world.advancedRendering.godRays.samples"

tests/domain/world-settings.test.ts

@@ -178,7 +178,7 @@ describe("world settings helpers", () => {
     const left = createDefaultWorldSettings();
     const right = cloneWorldSettings(left);
 
-    right.advancedRendering.godRays.enabled = true;
+    right.advancedRendering.godRays.sourceSize += 0.25;
 
     expect(areWorldSettingsEqual(left, right)).toBe(false);
   });