Add VACE Merge Back node for splicing VACE output into original video

Adds a new node that reconstructs full-length video by splicing VACE
sampler output back into the original clip at the trim positions. Supports
optical flow, alpha, and hard-cut blending at context/generated seams.
Also adds trim_start/trim_end INT outputs to VACESourcePrep.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/js/vace_widgets.js

@@ -76,3 +76,63 @@ app.registerExtension({
         updateVisibility(modeWidget.value);
     },
 });
+
+app.registerExtension({
+    name: "VACE.MergeBack.SmartDisplay",
+    nodeCreated(node) {
+        if (node.comfyClass !== "VACEMergeBack") return;
+
+        const methodWidget = node.widgets.find(w => w.name === "blend_method");
+        if (!methodWidget) return;
+
+        function toggleWidget(widget, show) {
+            if (!widget) return;
+            if (!widget._origType) widget._origType = widget.type;
+            widget.type = show ? widget._origType : "hidden";
+        }
+
+        function updateVisibility(method) {
+            const showBlend = method !== "none";
+            const showOf = method === "optical_flow";
+            toggleWidget(node.widgets.find(w => w.name === "blend_frames"), showBlend);
+            toggleWidget(node.widgets.find(w => w.name === "of_preset"), showOf);
+            node.setSize(node.computeSize());
+            app.graph.setDirtyCanvas(true);
+        }
+
+        const descriptor = Object.getOwnPropertyDescriptor(methodWidget, "value") ||
+            { configurable: true };
+        const hasCustomAccessor = !!descriptor.get;
+
+        if (!hasCustomAccessor) {
+            let _value = methodWidget.value;
+            Object.defineProperty(methodWidget, "value", {
+                get() { return _value; },
+                set(v) {
+                    _value = v;
+                    updateVisibility(v);
+                },
+                configurable: true,
+            });
+        } else {
+            const origGet = descriptor.get;
+            const origSet = descriptor.set;
+            Object.defineProperty(methodWidget, "value", {
+                get() { return origGet.call(this); },
+                set(v) {
+                    origSet.call(this, v);
+                    updateVisibility(v);
+                },
+                configurable: true,
+            });
+        }
+
+        const origCallback = methodWidget.callback;
+        methodWidget.callback = function(value) {
+            updateVisibility(value);
+            if (origCallback) origCallback.call(this, value);
+        };
+
+        updateVisibility(methodWidget.value);
+    },
+});