iris: eliminate retained-frame heap churn

docs/PLAN.md

@@ -819,7 +819,9 @@ reads it during `draw` and calls `request_next_frame` while it remains active.
 Iris stages the requesting widget's invalidation until the current retained
 redraw has finished, then the host schedules the next platform frame before
 renderer update or swapchain acquisition. `Widget::draw` still returns unit,
-and `Widget` has no tick callback.
+and `Widget` has no tick callback. Retained draw records alternate between two
+buffers for children, primitives, textures, and paints, so steady frame-driven
+redraws reuse their storage and matching resource handles.
 
 **Iris ships no fonts, and font families are application-named strings**
 (2026-09-12). Applications register their own font bytes on `Ui` and select