feat: skip track forward on result overlap, not class presence (REQ-189)

Frame holding the tracked class is now annotated like any other; the run's
result is dropped only when it intersects a same-class shape already there.

frontend/src/components/Filmstrip.jsx

@@ -1,7 +1,7 @@
 import React from 'react'
 import { api } from '../api'
 
-export default function Filmstrip({ frames, index, onSelectIndex, stripRef, markedIds, markLabel = 'already has it' }) {
+export default function Filmstrip({ frames, index, onSelectIndex, stripRef, markedIds, markLabel = 'overlaps an existing shape' }) {
   return (
     <div className="filmstrip" ref={stripRef}>
       {frames.map((item, position) => {

frontend/src/pages/ReviewPage.jsx

@@ -418,22 +418,23 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
       const marked = new Set()
       for (let i = 0; i < ahead.length; i++) {
         const targetFrame = ahead[i]
-        // A frame that already holds this class is left alone — hidden or
-        // dimmed shapes count. It is reported, never silently passed over.
-        if ((targetFrame.class_ids ?? []).includes(current.class_id)) {
-          skipped++
-          marked.add(targetFrame.id)
-          continue
-        }
         try {
-          await api.assist(targetFrame.id, { box, class_id: current.class_id }, { signal: controller.signal })
+          // dedupe makes the backend drop a result that lands on a shape of
+          // this class already on the frame — that is the same object twice —
+          // and answer { skipped: true } without writing it.
+          const created = await api.assist(
+            targetFrame.id,
+            { box, class_id: current.class_id, dedupe: true },
+            { signal: controller.signal },
+          )
+          if (created?.skipped) {
+            skipped++
+            marked.add(targetFrame.id)
+            continue
+          }
           tracked++
           patchFrameLocally(targetFrame.id, {
             annotation_count: (targetFrame.annotation_count ?? 0) + 1,
-            // Keeps the skip check honest for a second run in this session.
-            class_ids: (targetFrame.class_ids ?? []).includes(current.class_id)
-              ? targetFrame.class_ids
-              : [...(targetFrame.class_ids ?? []), current.class_id],
           })
         } catch (exc) {
           if (controller.signal.aborted) {
@@ -448,7 +449,7 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
         setSkippedLabel(className)
       }
       const parts = [`Tracked ${tracked} of ${ahead.length}`]
-      if (skipped) parts.push(`skipped ${skipped} already had ${className}`)
+      if (skipped) parts.push(`skipped ${skipped} overlapping ${className}`)
       if (failures.length) parts.push(failures.join(' · '))
       if (tracked === 0 || skipped || failures.length) setError(parts.join(' — '))
     } finally {
@@ -780,7 +781,7 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
           <Filmstrip
             frames={frames} index={index} onSelectIndex={setIndex} stripRef={stripRef}
             markedIds={skippedIds}
-            markLabel={`already has ${skippedLabel}`}
+            markLabel={`overlaps an existing ${skippedLabel}`}
           />
         </div>