iris: eliminate retained-frame heap churn

docs/LAYOUT.md

@@ -215,6 +215,12 @@ widget observed during that same draw; the next draw replaces the list, so a
 dependency disappears as soon as the widget stops reading it. Both fields
 have `ActiveData`'s existing lifecycle through `remove`/`remove_rec`.
 
+Retained draw output uses two buffers per collection. A redraw clears and
+fills the spare child, primitive, texture, and paint buffers while consuming
+the current buffers for reuse, then swaps their roles. Stable redraws therefore
+reuse vector capacity and move matching resource handles instead of allocating
+new collections or changing resource reference counts each frame.
+
 ### 6. Rejected alternatives
 
 - **A flat (non-chained) per-subtree offset table**, Iris's literal