iris: fold render state into Ui

docs/RUST.md

@@ -60,8 +60,9 @@ runs inside `cargo test`.
 
 1. **Headless, in-process, no compositor and no GPU -- the default.**
    `iris::harness` (`iris/src/harness.rs`), plus the fixture crate it
-   opens. `Harness::new(size, density)` builds an `Rsc`, a
-   `UiRenderState` and a state whose `FocusHost`/`OpenUrl` *record* what
+   opens. `Harness::new(size, density)` builds an `Rsc` whose `Ui` owns
+   the retained render state, and a state whose `FocusHost`/`OpenUrl`
+   *record* what
    the platform was asked for; `frame(t_ms)`/`frames_until(..)` run
    frames on a clock the test owns, and `replay(&TouchScript)` feeds a
    recorded gesture one sample at a time exactly as