docs/LAYOUT.md: masks reference a drawn primitive instead of copying a shape, and hit-testing applies the shape (Iris, 2026-09-07)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/RUST.md

@@ -60,8 +60,9 @@ closes it.
   recent log; a debug button copies it; Dev Updater reads it), write
   the decision in docs/DECISIONS.md, build it.
 - [ ] Masks with a shape -- docs/LAYOUT.md "Masks with a shape (decided
-  2026-09-07)". Rounded masks through the `Rect` SDF, chained and
-  multiplied, the container as the mask.
+  2026-09-07)". A mask references a primitive already drawn
+  (rect SDF, texture or glyph alpha), chained and multiplied; `.masked()`
+  points at the widget's own primitives; hit-testing applies the shape.
 - [ ] Compose app: the `Reversed range` crash in `ToolInput.highlighted`
   (docs/TODO.md). Main branch, not rustify.