Add scoped overlay hosts

docs/IRIS_TODO.md

@@ -30,11 +30,6 @@ Framework capabilities needed by `RUST.md`'s port plan:
   range. The shared `TextSelection` engine already computes the same geometry
   for selection highlights; expose one shared primitive rather than giving
   the app a second text-layout path.
-- [ ] **A modal/dialog primitive.** (**P1**, reused by **P3** and
-  **P5**.) Needed for the session settings dialog, `UsageDialog`'s
-  equivalent, and the delete-with-`deleteForeign` confirmation with its
-  toggle switch. Build once, wherever it is first needed, rather than
-  once per screen that wants one.
 - [ ] **A horizontal gauge/bar widget.** (**P1**.) For
   `SessionUsageBar`'s equivalent — a bounded fill reflecting a fraction,
   nothing fancier.