Move LazySpan rows through one retained offset

docs/IRIS_TODO.md

@@ -12,35 +12,6 @@ and six phone-report sections went on 2026-09-08 for that reason.
 
 ## Fix
 
-- [ ] **A row moving because the list grew should be one `move_offsets`
-  write, and today it is a redraw.** Found 2026-09-09 by
-  `scripts/rigs/ui-profile`'s `arena_churn` and left for whoever picks
-  this up next; the upload half of it is done and this is the layout
-  half.
-
-  The measurement. Over the bench fixture's 401 streamed deltas, the
-  instance arena uploads **71.9%** of itself per frame against a
-  **71.8%** floor -- those entries genuinely differ, so no amount of
-  better dirty-tracking touches it. The control that says it is wrong is
-  the fling phase on the same screen and the same content: it moves the
-  same primitives every frame and uploads **3.3%**, because a scroll
-  reaches `UiRenderState::mov` and writes one `move_offsets` delta for
-  the subtree (LAYOUT.md section 2) instead of rewriting every
-  primitive's absolute region.
-
-  The list is pinned to the newest end, so a growing reply pushes every
-  earlier row up. `draw_inner` now treats sub-pixel size differences as a
-  move and the span refactor removed nearly all repeated draws, but the
-  instance floor remains 71.8%. The remaining question is why those
-  translations still rewrite primitive regions instead of stopping at
-  the rows' move slots.
-
-  Done looks like: `arena_churn`'s `what_a_streamed_reply_uploads` shows
-  stream instances in the same range as the fling's, and its `whole`
-  column stops being the interesting one. The rig prints floor,
-  uploaded and whole per array precisely so this is checkable rather
-  than argued.
-
 - [ ] **Where the scroll *pin* lives.** The rest of "scrolling moves out
   of the list" landed on 2026-09-08 -- `List` is `LazySpan`, the physics
   and the gesture live in one `ScrollController`, `.scrollable()` is the