Add underwater fog functionality

tests/unit/underwater-fog.test.ts

@@ -0,0 +1,51 @@
+import { describe, expect, it } from "vitest";
+
+import { resolveUnderwaterFogState } from "../../src/runtime-three/underwater-fog";
+
+describe("resolveUnderwaterFogState", () => {
+  const runtimeScene = {
+    volumes: {
+      water: [
+        {
+          brushId: "water-1",
+          center: { x: 0, y: 0, z: 0 },
+          rotationDegrees: { x: 0, y: 25, z: 0 },
+          size: { x: 10, y: 4, z: 8 },
+          colorHex: "#347aa0",
+          surfaceOpacity: 0.72,
+          waveStrength: 0.4
+        }
+      ],
+      fog: []
+    }
+  };
+
+  it("returns fog tint and density from the containing water volume", () => {
+    const fogState = resolveUnderwaterFogState(runtimeScene, {
+      cameraSubmerged: true,
+      eyePosition: { x: 0.3, y: 1.1, z: 0.2 }
+    });
+
+    expect(fogState).not.toBeNull();
+    expect(fogState?.colorHex).toBe("#347aa0");
+    expect(fogState?.density ?? 0).toBeCloseTo(0.03352, 5);
+  });
+
+  it("does not enable fog when the camera is not submerged", () => {
+    expect(
+      resolveUnderwaterFogState(runtimeScene, {
+        cameraSubmerged: false,
+        eyePosition: { x: 0.3, y: 1.1, z: 0.2 }
+      })
+    ).toBeNull();
+  });
+
+  it("does not enable fog for points outside the water volume", () => {
+    expect(
+      resolveUnderwaterFogState(runtimeScene, {
+        cameraSubmerged: true,
+        eyePosition: { x: 20, y: 1.1, z: 0.2 }
+      })
+    ).toBeNull();
+  });
+});