iris: record the streamed-row redraw as an open item

arena_churn says a streamed frame uploads 72.7% of the instance arena
and that this is the floor, against 3.3% for a fling over the same
content -- so the rows are being redrawn where a scroll would write one
move_offsets delta. The upload half landed in 3c7d3db; this is the
layout half, with the measurement, the control that makes it convincing,
and where to look first.

docs/IRIS_TODO.md

@@ -12,6 +12,41 @@ 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 **72.7%** of itself per frame, and that number
+  *is* the 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.
+
+  What is different about the streaming path. The list is pinned to the
+  newest end, so a growing reply pushes every row above it up by the
+  amount the last row grew. That is a translation of an already-drawn
+  subtree -- exactly what `move_offsets` is for -- but it arrives as a
+  new offered region per row, and `draw_inner`'s fast path only takes
+  `mov` when `active.region.size() == region.size()`. Worth checking
+  first: whether `LazySpan::place` is offering each row a region whose
+  *size* differs (it computes `edges(height)` fresh each frame, so an
+  identical height should compare equal -- unless a float differs in the
+  last bit, which the `// TODO: epsilon?` beside that comparison already
+  suspects), or whether something upstream marks the rows dirty so the
+  fast path is skipped entirely.
+
+  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