feat: chain track 5 frames so a moving object is re-seeded (REQ-189)

trackForward sent the identical box to all five target frames. On batch 19
(extracted at 0.5 fps - one frame every 2 s) a truck entering the frame
outruns a static seed, so the action either missed it or grabbed the wrong
part.

- geometry handling lifted to module-level shapeBox(), reused for the drawn
  shape and for each assist result
- first frame still uses the drawn box exactly; frames 2-5 are seeded with
  the previous run's shape grown x1.5 about its centre (SEED_GROWTH)
- a frame SAM3 refuses keeps the last good seed for the next attempt
- REQ-189 rewritten to the chaining contract, including the accepted cost:
  a bad read can carry forward; this is not tracking, no motion model

frontend/src/pages/ReviewPage.jsx

@@ -9,6 +9,37 @@ import ReviewSidebar from '../components/ReviewSidebar'
 import ExemplarFilterPanel from '../components/ExemplarFilterPanel'
 import useExemplarPool from '../hooks/useExemplarPool'
 
+// How much bigger the seed box gets before each frame after the first, so the
+// object has room to have moved. Hand-tuned, not learned (REQ-189).
+const SEED_GROWTH = 1.5
+
+// SAM3 assist wants one normalized [x0, y0, x1, y1] box (REQ-043). A bbox
+// shape already is that; a polygon is reduced to its bounding box.
+function shapeBox(geometry) {
+  const pts = geometry?.points
+  if (geometry?.type === 'bbox' && pts?.length === 4) return pts
+  if (geometry?.type === 'polygon' && pts?.length) {
+    let minX = 1, minY = 1, maxX = 0, maxY = 0
+    for (const [x, y] of pts) {
+      if (x < minX) minX = x
+      if (x > maxX) maxX = x
+      if (y < minY) minY = y
+      if (y > maxY) maxY = y
+    }
+    return [minX, minY, maxX, maxY]
+  }
+  return null
+}
+
+function growBox([x0, y0, x1, y1], k) {
+  const cx = (x0 + x1) / 2
+  const cy = (y0 + y1) / 2
+  const hw = ((x1 - x0) * k) / 2
+  const hh = ((y1 - y0) * k) / 2
+  const clamp = (v) => Math.min(1, Math.max(0, v))
+  return [clamp(cx - hw), clamp(cy - hh), clamp(cx + hw), clamp(cy + hh)]
+}
+
 export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }) {
   const [batch, setBatch] = useState(null)
   const [project, setProject] = useState(null)
@@ -367,23 +398,8 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
       return
     }
 
-    // SAM3 assist wants one normalized [x0, y0, x1, y1] box (REQ-043). A bbox
-    // shape already is that; a polygon is reduced to its bounding box.
-    const geometry = current.geometry
-    let box = null
-    if (geometry?.type === 'bbox' && geometry.points?.length === 4) {
-      box = geometry.points
-    } else if (geometry?.type === 'polygon' && geometry.points?.length) {
-      let minX = 1, minY = 1, maxX = 0, maxY = 0
-      for (const [x, y] of geometry.points) {
-        if (x < minX) minX = x
-        if (x > maxX) maxX = x
-        if (y < minY) minY = y
-        if (y > maxY) maxY = y
-      }
-      box = [minX, minY, maxX, maxY]
-    }
-    if (!box) {
+    const firstBox = shapeBox(current.geometry)
+    if (!firstBox) {
       setError('That shape has no usable box — Track 5 Frames needs one.')
       return
     }
@@ -397,12 +413,19 @@ export default function ReviewPage({ batchId: rawBatchId, projectId, onProject }
     setError('')
     try {
       let tracked = 0
+      let seed = firstBox
       const failures = []
       for (let i = 0; i < ahead.length; i++) {
         const targetFrame = ahead[i]
+        // The first frame uses the drawn box untouched. After that the object
+        // has had time to move, so the last result seeds a wider box.
+        const seedBox = i === 0 ? seed : growBox(seed, SEED_GROWTH)
         try {
-          await api.assist(targetFrame.id, { box, class_id: current.class_id })
+          const created = await api.assist(targetFrame.id, {
+            box: seedBox, class_id: current.class_id,
+          })
           tracked++
+          seed = shapeBox(created?.geometry) ?? seed
           patchFrameLocally(targetFrame.id, { annotation_count: (targetFrame.annotation_count ?? 0) + 1 })
         } catch (exc) {
           failures.push(`frame ${index + i + 2}: ${exc.message}`)