Remove unused background overlay and update tests

tests/unit/world-background-renderer.test.ts

@@ -0,0 +1,73 @@
+import { Texture } from "three";
+import { describe, expect, it } from "vitest";
+
+import { createDefaultWorldSettings } from "../../src/document/world-settings";
+import { resolveWorldEnvironmentState } from "../../src/rendering/world-background-renderer";
+
+describe("resolveWorldEnvironmentState", () => {
+  it("keeps the authored day environment when no night overlay is active", () => {
+    const world = createDefaultWorldSettings();
+    const dayTexture = new Texture();
+    world.background = {
+      mode: "image",
+      assetId: "asset-day-sky",
+      environmentIntensity: 0.65
+    };
+
+    expect(
+      resolveWorldEnvironmentState(world.background, dayTexture, null)
+    ).toEqual({
+      texture: dayTexture,
+      intensity: 0.65
+    });
+  });
+
+  it("hands the environment off through zero intensity during a day-to-night crossfade", () => {
+    const world = createDefaultWorldSettings();
+    const dayTexture = new Texture();
+    const nightTexture = new Texture();
+    world.background = {
+      mode: "image",
+      assetId: "asset-day-sky",
+      environmentIntensity: 0.4
+    };
+
+    expect(
+      resolveWorldEnvironmentState(world.background, dayTexture, {
+        texture: nightTexture,
+        opacity: 0.25,
+        environmentIntensity: 0.8
+      })
+    ).toEqual({
+      texture: dayTexture,
+      intensity: 0.2
+    });
+
+    expect(
+      resolveWorldEnvironmentState(world.background, dayTexture, {
+        texture: nightTexture,
+        opacity: 0.75,
+        environmentIntensity: 0.8
+      })
+    ).toEqual({
+      texture: nightTexture,
+      intensity: 0.4
+    });
+  });
+
+  it("fades the night environment in when the authored day background has no image environment", () => {
+    const world = createDefaultWorldSettings();
+    const nightTexture = new Texture();
+
+    expect(
+      resolveWorldEnvironmentState(world.background, null, {
+        texture: nightTexture,
+        opacity: 0.5,
+        environmentIntensity: 0.7
+      })
+    ).toEqual({
+      texture: nightTexture,
+      intensity: 0.35
+    });
+  });
+});