revert: undo chained/seed-grown track 5 frames, back to b71f1d4

Reverts 1530bbc and 9c95a4a. Track 5 Frames sends the drawn box unchanged to
all five frames - no chaining, no growth. Both were rejected: chaining let a
wrong read on one frame poison the next, and growth was an attempt to follow a
moving object that the user did not want.

REQ-189, ui-spec and tasks.md return to the b71f1d4 wording.

docs/tasks.md

@@ -1405,23 +1405,6 @@ read it.
    (accent border + glow) — no CSS added; **browser eyeball still owed by the user**
    (375 px 2-col wrap, long class names).
 
-## Task — Track 5 Frames follows a moving object (REQ-189) `[DONE]`
-
-1. Root cause: `trackForward` sent the **same box** to all 5 frames, and batch 19 was
-   extracted at **fps 0.5** (one frame every 2 s, `batches.fps`) — a truck entering frame
-   outruns a static seed. First attempt chained each run's result into the next frame's seed;
-   that let a wrong read on one frame carry into the following ones, which the user rejected
-   as poisoning. Final shape: runs are **independent again**, every frame seeded from the
-   drawn shape, frames 2–5 grown ×1.5 about its centre (`SEED_GROWTH`, `growBox`) for motion
-   tolerance. Geometry handling moved to module-level `shapeBox()`.
-   → verify: **[DONE]** `npm run build` green; no frame's seed derives from a previous result
-   (`seed` reassignment removed, grep confirms `firstBox` is the only seed source); first frame
-   un-grown so the stationary case keeps exact precision; growth ×1.5 total size (not 2×);
-   busy guard still blocks double-writes. **Moving-object behaviour needs the user's GPU
-   run** — and without chaining, a fast-moving truck may still leave the seed window; fix is
-   re-extract at 2 fps (*Trim → frames per second*) or a video-propagation requirement
-   (not yet written; `sam3_video_inference.py` already has `propagate_in_video`).
-
 ## Task — Track 5 Frames works at all (REQ-189) `[DONE]`
 
 1. `ReviewPage.jsx:trackForward` read `geometry.coordinates` — a key the backend never emits