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

docs/requirements.md

@@ -170,13 +170,17 @@ 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, 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.
+  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 **chained, not
+  independent**: the first frame gets the drawn box untouched, and after each successful run
+  that run's resulting shape seeds the next frame, grown about its centre by ×1.5 so the
+  object has room to have moved. A frame SAM3 refuses (nothing inside the box) is reported
+  and the run continues from the last good seed. Nothing selected, an unusable shape, or the
+  last frame says so instead of doing nothing. Known cost of chaining: a wrong read on one
+  frame can carry into the ones after it — the result banner reports only what happened. 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 do
+  not share state between invocations.
 - **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