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.

docs/RUST.md

@@ -678,8 +678,8 @@ it is both. This is an `app-rust` defect, not an iris framework item.
             `AI_SANDBOX_BIG_MB` and `--delay`.
       - [ ] **P1d — images, the session settings dialog, attachments,
             usage bar.** `SessionImage` thumbnails (the scaled image
-            widget), the modal primitive and `SessionSettingsDialog`/
-            `UsageDialog`, `PendingAttachments` over the attachments
+            widget), the existing overlay facility and
+            `SessionSettingsDialog`/`UsageDialog`, `PendingAttachments` over the attachments
             route (`api.rs` gap), `SessionUsageBar` (the gauge widget).
       - [ ] **P1e — keyboard and insets behaviours** from AGENTS.md's
             "Things that have bitten", re-verified on the phone build:
@@ -715,11 +715,11 @@ it is both. This is an `app-rust` defect, not an iris framework item.
       **iris widgets missing, → `IRIS_TODO.md`'s "Build (for the port)"
       section**: the distance-to-unloaded-edge query P1c needs for its
       viewport-sized history cushion; per-range text backgrounds for inline
-      code; a fitted image widget; one modal/dialog primitive reused by the
-      settings and usage dialogs; and a horizontal gauge for
-      `SessionUsageBar`. Tappable links already exist. Row accessibility
-      names are app content applied through iris's existing `.label()` API,
-      not a missing framework widget.
+      code; a fitted image widget; and a horizontal gauge for
+      `SessionUsageBar`. The settings and usage dialogs can use Iris's
+      existing local overlay hosts. Tappable links already exist. Row
+      accessibility names are app content applied through iris's existing
+      `.label()` API, not a missing framework widget.
 
       **Pass condition**: `app/ui-sandbox.sh`'s fixtures driven by
       `ui-trace record --do "tap '<label>'"` — a session with the big
@@ -790,7 +790,7 @@ it is both. This is an `app-rust` defect, not an iris framework item.
       dimmed, drained of colour and labelled with the operation in progress
       without blocking the list's scroll — so it belongs in `src/ui`, not
       iris. `uniqueItems` is app logic there too. Iris itself still needs
-      the modal primitive P1 flagged and a toggle switch for the
+      a toggle switch for the
       delete-with-`deleteForeign` flow.
 
       **Pass condition**: `ui-trace` tap-by-name on all four tabs against