fix: unchain track 5 frames so one bad read cannot poison the rest

The chained attempt (1530bbc) fed each run's result into the next frame's
seed, so a wrong read on one frame carried into the ones after it. Runs are
independent again: every frame is seeded from the drawn shape, frames 2-5
with it grown x1.5 about its centre for motion tolerance. REQ-189, ui-spec
and tasks.md updated to the independent contract.

frontend/src/pages/ReviewPage.jsx

@@ -9,8 +9,9 @@ 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).
+// How much bigger the seed box gets for each frame after the first, so the
+// object has room to have moved. Hand-tuned, not learned (REQ-189). The box
+// always derives from the drawn shape — never from a previous run's result.
 const SEED_GROWTH = 1.5
 
 // SAM3 assist wants one normalized [x0, y0, x1, y1] box (REQ-043). A bbox
@@ -413,19 +414,19 @@ 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)
+        // Every frame is seeded from the drawn box, never from a previous
+        // result, so a bad read on one frame cannot carry into the next.
+        // Frames 2-5 get it grown about its centre — the object has had
+        // time to move by then.
+        const seedBox = i === 0 ? firstBox : growBox(firstBox, SEED_GROWTH)
         try {
-          const created = await api.assist(targetFrame.id, {
+          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}`)