refactor(cine): fold tool-slice fallback into getRenderSlice

getRenderSlice now takes an optional toolSlice param; the three tool
widgets that previously computed `tool?.slice ?? slice.value` inline
just pass both to the helper instead.

Author: PaulHax

src/components/tools/polygon/PolygonWidget2D.vue

@@ -135,11 +135,10 @@ export default defineComponent({
     widget.setManipulator(manipulator);
 
     watchEffect(() => {
-      const semantic = tool.value?.slice ?? slice.value ?? 0;
       updatePlaneManipulatorFor2DView(
         manipulator,
         viewDirection.value,
-        getRenderSlice(imageId.value, semantic),
+        getRenderSlice(imageId.value, slice.value, tool.value?.slice),
         imageMetadata.value
       );
     });

src/components/tools/rectangle/RectangleWidget2D.vue

@@ -123,11 +123,10 @@ export default defineComponent({
     widget.setManipulator(manipulator);
 
     watchEffect(() => {
-      const semantic = tool.value?.slice ?? slice.value;
       updatePlaneManipulatorFor2DView(
         manipulator,
         viewDirection.value,
-        getRenderSlice(imageId.value, semantic),
+        getRenderSlice(imageId.value, slice.value, tool.value?.slice),
         imageMetadata.value
       );
     });

src/components/tools/ruler/RulerWidget2D.vue

@@ -122,11 +122,10 @@ export default defineComponent({
     widget.setManipulator(manipulator);
 
     watchEffect(() => {
-      const semantic = ruler.value?.slice ?? slice.value;
       updatePlaneManipulatorFor2DView(
         manipulator,
         viewDirection.value,
-        getRenderSlice(imageId.value, semantic),
+        getRenderSlice(imageId.value, slice.value, ruler.value?.slice),
         imageMetadata.value
       );
     });

src/core/cine/getRenderSlice.ts

@@ -9,9 +9,14 @@ import { isCineImage } from './isCineImage';
 // [0, numberOfFrames-1], but the underlying vtkImageData has only one Z slice
 // at index 0. Anything that pokes a VTK slice mapper or 2D widget plane needs
 // the render slice (always 0 for cine).
+//
+// `toolSlice` is an optional per-tool override (e.g. a ruler pinned to a
+// specific slice) — when defined it wins over the view's current slice.
 export function getRenderSlice(
   imageID: Maybe<string>,
-  semanticSlice: number
+  viewSlice: Maybe<number>,
+  toolSlice?: Maybe<number>
 ): number {
-  return isCineImage(imageID) ? 0 : semanticSlice;
+  if (isCineImage(imageID)) return 0;
+  return toolSlice ?? viewSlice ?? 0;
 }