Move LazySpan rows through one retained offset

AGENTS.md

@@ -446,16 +446,17 @@ Each exists because something was invisible without it.
   are only reusable next frame and provisional layout nested -- grew
   the arena to **127,443 slots for 11,569 live primitives**; and nothing
   tracked *which* entries changed. Now: the stream arena is 11,569 slots
-  for 11,569 live, and every array uploads within a hair of its floor
-  (fling instances 1.0% against a 0.9% floor, stream instances 71.9%
-  against 71.8%, and stream glyphs 0.6% against 0.6%). The CPU half improved with it, since the freeing and renumbering
+  for 11,569 live, and every array uploads within a hair of its floor.
+  The CPU half improved with it, since the freeing and renumbering
   went away: a streamed frame is p50 1.39ms, from 2.20ms.
 
-  **Stream instances sit at 71.9%, against a 71.8% floor, and dirty
-  tracking is not the defect here.** The list is pinned to the newest end, so a growing reply moves
-  every row, and a row's instances carry an absolute region. That is a
-  `move_offsets` write the layout is not making -- the next thing to look
-  at, and a layout question rather than an upload one.
+  The remaining layout cost was then removed at the framework boundary:
+  `Painter::set_child_offset` gives a container one retained coordinate slot
+  for its child subtree, and `LazySpan` keeps row boxes stable behind it.
+  Pinned growth now uploads instances at **2.9% against a 2.9% floor**, from
+  71.9% against 71.8%; p50 instance upload is **1,728 bytes**, from 176,496.
+  `Primitives` also cancels dirty marks for provisional writes restored before
+  upload, so CPU-only layout states never become GPU work.
 - **The emulator is a GLES rig, deliberately** (Iris, 2026-09-08;
   docs/RUST.md). Its guest has no hardware Vulkan -- only SwiftShader
   in software -- while its GLES *is* the host's real GPU through virgl at