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/ui-spec.md

@@ -937,7 +937,7 @@ worked". The jump happens once per load, and again after an auto-label job finis
 ### 5.2 Data model in play
 
 ```
-frame       { id, idx, filename, width, height, review_status, annotation_count, class_ids }
+frame       { id, idx, filename, width, height, review_status, annotation_count }
 annotation  { id, class_id, geometry, score, source: "auto"|"manual" }
 batch       { …, review: { pending, approved, rejected }, annotation_count, status }
 ```
@@ -1036,9 +1036,12 @@ an "asking SAM3…" indicator ·
   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) followed by `skipped K already had <class>` and the per-frame
-  failures. Skipped frames are those that already hold a shape of the tracked class (hidden or
-  dimmed counts); they get a dot on their filmstrip thumb and the thumb's tooltip says why. The
+  (M = frames actually ahead) followed by `skipped K overlapping <class>` and the per-frame
+  failures. Every frame ahead runs SAM3; a frame is skipped only when its **result** lands on a
+  shape of the tracked class already there (any intersection — hidden or dimmed counts), which
+  means a frame holding the tracked class elsewhere still gets its own second box. Skipped frames
+  get a dot on their filmstrip thumb and the thumb's tooltip says `overlaps an existing <class>`.
+  The
   "asking SAM3…" indicator becomes a **Cancel** button while a run is in flight (Cancel stops the
   remaining frames; the frame already in flight can still be saved by the server, and the banner
   says so). A second