Move LazySpan rows through one retained offset

docs/RUST.md

@@ -670,8 +670,10 @@ The trap that only the rig could have caught: writing an entry is not the
 same as changing it. Recycling rewrote every glyph of every moved row with
 identical bytes, marking 73% of the glyph array against 0.6% genuinely
 changed. `PrimitiveVec::set` and `Primitives::set_instance` compare before
-marking, and `arena_churn` prints both numbers so the gap cannot reopen
-unnoticed.
+marking. Layout can also write a provisional instance and restore it within
+one frame; `Primitives` remembers the pre-frame bytes and cancels that dirty
+bit when the GPU-visible result is unchanged. `arena_churn` prints both
+numbers so either gap cannot reopen unnoticed.
 
 **Layout has no measurement mode.** A widget is drawn provisionally only
 when its size cannot be known yet, and that retained drawing is moved into
@@ -685,12 +687,16 @@ Measured over the fixture's 401 streamed events: the busiest frame makes
 Streamed-frame CPU p50 is 0.35ms, from 1.18ms before this layout change.
 Arena size and upload floors are unchanged.
 
-**What is left, and it is a layout question rather than an upload one.**
-Stream instances upload 71.9%, against a 71.8% floor: the list is pinned to
-the newest end, so a growing reply moves every row, and a row's instances
-carry an absolute region. Moving a subtree is supposed to be one
-`move_offsets` write (LAYOUT.md section 2); something on this path is
-redrawing instead.
+Pinned growth now uses the same subtree translation as scrolling. A
+container can retain a child-coordinate move slot through
+`Painter::set_child_offset`; `LazySpan` keeps retained rows in stable local
+boxes and changes that one slot when its anchor moves. It still walks the
+visible run to virtualise it, but unchanged rows no longer acquire new
+absolute primitive regions. Over the fixture's 401 streamed events, instance
+upload is **2.9% against a 2.9% floor**, from 71.9% against 71.8%; median
+instance bytes per frame are **1,728**, from 176,496. This is framework
+layout/rendering behaviour and the transcript screen contains no special
+case for it.
 
 ### The Android release profile is `opt-level = 3`, not `"s"` (2026-09-09)