Move LazySpan rows through one retained offset

docs/LAYOUT.md

@@ -94,6 +94,15 @@ resolved in the vertex shader.**
 - `mov(id, delta)` becomes: look up `id`'s slot, write
   `move_offsets[slot].delta += delta`. One write — no primitive touched, no
   recursion, since descendants already reference this slot transitively.
+- A container may also retain one optional **child-coordinate slot** between
+  its own slot and every direct child's slot. `Painter::set_child_offset`
+  creates that boundary before the first child is drawn and can update it
+  after measuring a child on later redraws. The container's own primitives,
+  hit region and mask stay fixed; its whole child subtree moves through one
+  write and every existing GPU, hit-test and accessibility chain sees the
+  same result. `LazySpan` uses this while still walking visible rows for
+  virtualisation: row boxes stay in stable local coordinates and the shared
+  boundary carries the changing screen translation.
 - `shader.wgsl`'s vertex stage, after computing `top_left`/`bot_right` in
   pixels (after `:106`, before the clip-space divide at `:113`), walks
   `move_idx → move_offsets[i].parent` for a bounded number of steps (a
@@ -120,6 +129,12 @@ for a resize that changes a region's `rel` component (a genuine reflow,
 §3) and for a size-independent widget's resize (§3), where the content's
 shape doesn't change and one field write already suffices.
 
+Provisional layout can still write an instance at an intermediate position
+and restore it before upload. `Primitives::set_instance` remembers the value
+at the first write in a frame and clears the dirty bit when the final bytes
+match it. The GPU therefore observes final layout state, not CPU-only
+measurement work.
+
 ### 2b. Two more readers of "where is this widget," and masks
 
 Moving the offset into the vertex shader means `ActiveData.region` is no