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/ui-spec.md

@@ -989,21 +989,13 @@ previous geometry** and shows the error. Same optimistic-with-rollback pattern f
 - *Copy Prev* copies every annotation from frame `n-1` onto this frame (one POST per shape);
   disabled at index 0 or when the previous frame is empty.
 - *Track 5 Frames* takes the selected shape's bounding box and runs SAM3 assist on the next 5
-  frames with it — the first with that box exactly, the rest with it grown (see below). Works
-  for both geometry types: a bbox shape supplies its own box, a polygon
+  frames with it. Works for both geometry types: a bbox shape supplies its own box, a polygon
   is reduced to its bounding box. Nothing selected, an unusable shape, or the last frame
   says so in the error slot instead of silently doing nothing; a frame SAM3 refuses (nothing
   inside the box) does not stop the rest — the banner then **starts** with `Tracked N of M`
   (M = frames actually ahead, ≤ 5) followed by the per-frame failures. A second press while
   a run is in flight is ignored. Shapes are written `source=manual`, exactly as a hand-drawn
-  one.
-  Runs are **independent**: every frame is seeded from the drawn shape, never from another
-  run's result, so a wrong read on one frame cannot poison the ones after it. The first frame
-  uses the drawn box exactly; frames 2–5 use it grown ×1.5 about its centre so a moving object
-  still fits the seed window. A refused frame does not stop the rest. Because nothing carries
-  between frames there is no size/position readjustment — a fast-moving object can still
-  outrun the seed; only denser frame extraction (higher fps) or real video propagation fixes
-  that (REQ-189).
+  one (REQ-189).
 
 **Quick reclass bar** appears whenever a shape is selected: one button per class
 (`[n] name`, class-coloured) plus *Delete [Del]*.