Crossfade between palettes instead of flashing empty

Switching source kept nothing on screen until the new one drew. The old
frame now stays until the first new frame lands, then fades out over it
in 150 ms.

Model used: Claude Opus 5.5

core/src/test/java/art/arcane/iris/studio/map/FrameCrossfadeTest.java

@@ -0,0 +1,62 @@
+package art.arcane.iris.studio.map;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+
+public final class FrameCrossfadeTest {
+    private final FrameCrossfade crossfade = new FrameCrossfade(() -> {
+    });
+
+    @Test
+    public void heldFrameStaysFullyVisibleUntilTheNextFrameArrives() {
+        NoiseRenderCoordinator.Result frame = frame();
+
+        crossfade.hold(frame);
+
+        assertSame(frame, crossfade.outgoing());
+        assertEquals(1F, crossfade.outgoingOpacity(), 0F);
+    }
+
+    @Test
+    public void heldFrameFadesOnceTheNextFrameArrives() throws InterruptedException {
+        crossfade.hold(frame());
+        crossfade.reveal();
+        Thread.sleep(20L);
+
+        assertTrue(crossfade.outgoingOpacity() < 1F);
+    }
+
+    @Test
+    public void switchingAgainBeforeAnythingDrewKeepsTheFrameHeld() {
+        NoiseRenderCoordinator.Result frame = frame();
+        crossfade.hold(frame);
+
+        crossfade.hold(null);
+
+        assertSame(frame, crossfade.outgoing());
+    }
+
+    @Test
+    public void frameWithoutAHeldFrameHasNothingToFade() {
+        crossfade.reveal();
+
+        assertNull(crossfade.outgoing());
+    }
+
+    @Test
+    public void clearDropsTheHeldFrame() {
+        crossfade.hold(frame());
+
+        crossfade.clear();
+
+        assertNull(crossfade.outgoing());
+    }
+
+    private static NoiseRenderCoordinator.Result frame() {
+        return new NoiseRenderCoordinator.Result(null, null, 1D, 1L, 0D, 1D, 0.5D, 0L, 0L, 0L);
+    }
+}