feat: skip track forward on result overlap, not class presence (REQ-189)

Frame holding the tracked class is now annotated like any other; the run's
result is dropped only when it intersects a same-class shape already there.

docs/tasks.md

@@ -1511,7 +1511,8 @@ read it.
    slice bound is computed from `Number(trackFrames)` so a string can never concatenate into the
    slice end; `slice(index+1, index+1+N)` and `slice(index+1)` both bounded; cancel lands as
    `cancelled — frame n may still be saved by the server` and keeps the shapes already written;
-   skip check reads the `class_ids` array, not `annotation_count`; marks cleared at the start of
+   skip check reads the `class_ids` array, not `annotation_count` — *that rule was later replaced
+    by the post-run overlap test in the next task*; marks cleared at the start of
    every run; `Shift+T` deliberately **not** bound (a fat-fingered 500-frame GPU run is not worth
    a keystroke). Reviewer pass done — 4 findings fixed. **Still owed, user browser + GPU**: set
    N=3 → 3 shapes, reload → N remembered; to-end over the already-reviewed part of batch 19 →
@@ -1534,6 +1535,36 @@ read it.
    snapshot left as history); **SAM3 actually finding the object in the next 5 frames needs
    the user's GPU run** — select a shape, press `T`, expect a shape on each of the next 5.
 
+## Task — Track Forward skips on overlap, not on class presence (REQ-189 amended) `[DONE]`
+
+- The old rule skipped a frame *before* SAM3 ran when it already held a shape of the tracked
+  class, which contradicted REQ-189's own "each frame is an independent run": a second truck in
+  the same frame could never be annotated. Amended to a duplicate test **after** the run.
+- `backend/review.py:intersects()` — any intersection between two normalized boxes (shared area
+  > 0; edge touch is *not* an overlap), deliberately not IoU. `assist()` gained
+  `dedupe: bool = False`: once the GPU lock is released and right before `add()`, an existing
+  shape of the same class on that frame whose box intersects the result makes it answer
+  `{"skipped": True}` and write nothing — no create-then-delete, no transient row.
+  `backend/api/review.py:AssistRequest.dedupe` carries the flag; `False` keeps the single-shot
+  box-assist path byte-identical.
+- `frontend/src/pages/ReviewPage.jsx:trackForward` — pre-skip on `class_ids` and the local
+  `class_ids` union are gone, the call sends `dedupe: true`, and `{skipped: true}` lands in the
+  banner as `skipped K overlapping <class>` plus the filmstrip dot `overlaps an existing <class>`.
+- `backend/batches.py:frames()` no longer ships `class_ids` (it was added for the pre-skip and
+  died with it); `annotation_count` stays.
+- Docs: `requirements.md` REQ-189 skip sentence rewritten, `ui-spec.md` banner + thumb tooltip.
+- Cost accepted: a long run now pays a GPU call on **every** frame ahead, including frames that
+  end up skipped, where the old pre-skip made those free. Only worth layering a cheap seed-box
+  pre-skip back if that hurts.
+→ verify: **[DONE]** `uv run python` harness (mocked engine + temp PNG) — 6 `intersects`
+  cases (edge-touch false, partial/contained/corner true, disjoint false) and 5 `assist()` paths:
+  dedupe+overlap → `{"skipped": True}` with `add()` never called; other-class box → annotated;
+  empty frame → annotated; `dedupe=False` → annotated; polygon annotation → skipped.
+  `batches.frames()` run against the live DB (3737 rows, `class_ids` gone, `annotation_count`
+  kept); `npm run build` green; `git diff --check` clean. **Still worth a GPU run:** track into a
+  frame already holding an overlapping truck → banner `skipped K overlapping truck`, dot, count
+  unchanged, while a frame holding a truck elsewhere gets its own second box.
+
 ## Known open points
 
 - *Not closed by any task, by choice:* **any rebuild kills the running job.** Task 14's resume