iris: I4 -- accessibility names via AccessKit

Builds one flat AccessKit tree (iris_core::ui::access::AccessTree) from
iris's own widget tree: a synthetic Role::Window root with every named
widget as a direct child, names from the existing `.label()`, roles from
a new Widget::access_role() (default Unknown, TextEdit overrides to
TextInput/MultilineTextInput), bounds from UiRenderState::window_region
so a moved subtree reports where it actually is. Modular the way input's
sense registry is: Widgets gained one HashSet<WidgetId> ("named"),
populated only by .label()/set_label and drained by free_next (the
existing removal path), and AccessTree walks only that set -- a widget
nobody named costs it nothing. Updates only when the named set's name,
role or bounds actually changed, with a rebuild counter mirroring
take_counters (confirmed 1/0/1 across first-draw/unchanged/moved in
access_tests.rs).

Pushed through accesskit_winit on the desktop (DefaultApp::new now
creates the window hidden, builds the adapter, then shows it, per that
constructor's requirement) and accesskit_android on Android
(IrisViewPeer now implements AccessibilityNodeProvider). Both action
handlers are inert on purpose: AGENTS.md's tap-by-name is a real touch
at the node's bounds, not an AccessKit action request, so the ordinary
pointer path already answers it once bounds are right. E1's
detach-abort mitigation is carried into android/access.rs's
raise_if_enabled, which gates every QueuedEvents::raise on
AccessibilityManager.isEnabled().

tabs-ui's five switch buttons now carry .label()s matching their
on-screen text, giving both the desktop run and the emulator step real
names to find.

Verified on host: cargo fmt/build/clippy/test all clean (28 tests, 3
new), cargo ndk build+clippy clean for iris and iris-android-app,
run-headless.sh tabs --shot byte-identical to I2's prior screenshot
(27266 bytes). Not run: the emulator step (ui-trace tap-by-name against
iris-android-app), held by another session this pass -- exact commands
recorded in RUST.md's I4 box.

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

IRIS.md

@@ -8,6 +8,37 @@ capability that moved. Small and trivial changes do not go here.
 An entry gives the date, what changed, why, and a short before/after where
 it helps judge the change without the session that made it. Newest first.
 
+## 2026-09-05: accessibility names via AccessKit (RUST.md's I4)
+
+`.label()` (already in `trait_fns.rs`, previously unused anywhere in-tree)
+is now load-bearing: it's the one thing that puts a widget in the AccessKit
+tree `iris_core::ui::access::AccessTree` builds and both backends push
+out. A widget author who wants a control to be findable by name (and
+tappable by name, through `ui-trace`/a real screen reader) calls `.label()`
+on it; nothing else is required, and a widget nobody labels is invisible
+to this system at zero cost, not just zero UI.
+
+```rust
+let button = rect(Color::LIME)
+    .on(CursorSense::click(), move |_, rsc| { ... })
+    .label("Add task");   // now findable by uiautomator/AccessKit as "Add task"
+```
+
+Two new things a widget author might touch directly:
+
+- **`Widget::access_role(&self) -> accesskit::Role`**, default `Unknown`.
+  Override it if your widget has a real platform equivalent —
+  `TextEdit` now returns `TextInput`/`MultilineTextInput` by `EditMode`.
+  Only consulted for a widget that also has a `.label()`; an unlabelled
+  widget's `access_role` is never called.
+- **`Widgets::named() -> impl Iterator<Item = WidgetId>`** — every widget
+  with an explicit label, for anything else that wants to walk the same
+  set `AccessTree` does.
+
+Nothing about `Painter`, `draw`, or the layout/move machinery changed —
+this sits entirely beside them, reading `resolved_region`'s output rather
+than participating in producing it.
+
 ## 2026-09-05: `List`, a virtualised bottom-anchored list (RUST.md's I3)
 
 A new widget, `iris::widget::List` (`iris/src/widget/list.rs` -- read its