Add tests for computeSoundEmitterDistanceGain function

tests/domain/runtime-audio-system.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+
+import { computeSoundEmitterDistanceGain } from "../../src/runtime-three/runtime-audio-system";
+
+describe("computeSoundEmitterDistanceGain", () => {
+  it("keeps full volume near the emitter and eases smoothly to silence at max distance", () => {
+    expect(computeSoundEmitterDistanceGain(4, 6, 24)).toBe(1);
+    expect(computeSoundEmitterDistanceGain(6, 6, 24)).toBe(1);
+    expect(computeSoundEmitterDistanceGain(12, 6, 24)).toBeGreaterThan(2 / 3);
+    expect(computeSoundEmitterDistanceGain(18, 6, 24)).toBeLessThan(1 / 3);
+    expect(computeSoundEmitterDistanceGain(24, 6, 24)).toBe(0);
+    expect(computeSoundEmitterDistanceGain(30, 6, 24)).toBe(0);
+  });
+});