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/requirements.md

@@ -200,9 +200,12 @@ changes.
   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. A frame that **already holds a shape of the tracked class is skipped** — hidden or
-  dimmed shapes count as present — and the skip is reported in the result banner and marked
-  with a dot on that frame's filmstrip thumbnail, never silently passed over. A long run can be
+  nothing. Every frame ahead **runs** — a frame that already holds a shape of the tracked class
+  is still annotated when SAM3 finds a *different* object there. What is dropped is a
+  **duplicate**: a result that intersects an existing shape of the tracked class (any shared
+  area counts, hidden or dimmed shapes count) is not written, and that skip is reported in the
+  result banner and marked with a dot on that frame's filmstrip thumbnail, never silently passed
+  over. A long run can be
   **cancelled** mid-flight: cancelling stops the remaining frames, but a request already in
   flight is not aborted server-side and may still save its shape. The box is never grown and never reseeded from a result, so there is
   no size or position readjustment between frames: a fast-moving object can still leave the