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/requirements.md

@@ -170,20 +170,13 @@ changes.
   one object and the model produces its shape.
 - **REQ-189** — In the review editor, the user can push the **selected shape's** box forward
   with *Track 5 Frames* (`[T]`): the next five frames each get their own REQ-043 box-assist
-  run, and the shape is written as `source=manual`. Both geometry types work — a bbox shape
-  supplies its own box, a polygon is reduced to its bounding box. Runs are **independent**:
-  every frame is seeded from the drawn shape and never from another run's result, so a wrong
-  read on one frame cannot carry into the ones after it. The first frame uses the drawn box
-  untouched; frames 2–5 use it grown about its centre by ×1.5, giving a moving object room
-  to be inside the seed window. A frame SAM3 refuses (nothing inside the box) is reported and
-  the remaining frames still run. Nothing selected, an unusable shape, or the last frame says
-  so instead of doing nothing. Consequence accepted: without chaining there is no size or
-  position readjustment between frames, so a fast-moving object can still leave the seed
-  window — denser extraction (higher fps) or a video-propagation requirement (not yet
-  written) is the fix. This is
-  one-shot hand-off across the next frames, **not** propagation of drawn exemplars (which
-  stays a non-goal): nothing is tracked frame-to-frame with a motion model, and the runs
-  share no state between invocations.
+  run, seeded with that box, and the shape is written as `source=manual`. Both geometry types
+  work — a bbox shape supplies its own box, a polygon is reduced to its bounding box. Each
+  frame is an **independent** run: a frame SAM3 refuses (nothing inside the box) is reported
+  and the rest still run; nothing selected, an unusable shape, or the last frame says so
+  instead of doing nothing. This is one-shot hand-off across the next frames, **not**
+  propagation of drawn exemplars (which stays a non-goal): nothing is carried frame to frame,
+  and the runs do not depend on each other.
 - **REQ-044** — All annotations and review statuses are **persistent** — they survive a
   server restart, unlike today's in-memory sessions.
 - **REQ-045** — Review progress is visible (e.g. "120/300 reviewed"), and a batch can only