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

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