iris: move text state into shared resources

docs/PLAN.md

@@ -814,13 +814,18 @@ private implementations of that wake; applications do not define platform
 event types or manually request redraws.
 
 **Iris ships no fonts, and font families are application-named strings**
-(2026-09-12). Applications register their own font bytes on `Ui` before the
-first text shape and select them by the same string used by text widgets. The
-public boundaries accept `AsRef<str>`, so an application can use bare strings
-or put its own semantic enum in front of them without Iris hardcoding the
-roles. ai-app owns the `ai-app-icons` family, its Nerd Fonts subset, its
-codepoints, its license and the script that rebuilds it; the CSS generic names
-`sans-serif` and `monospace` continue to resolve through the platform.
+(2026-09-12). Applications register their own font bytes on `Ui` and select
+them by the same string used by text widgets. The public boundaries accept
+`AsRef<str>`, so an application can use bare strings or put its own semantic
+enum in front of them without Iris hardcoding the roles. Text buffers, layouts,
+and prepared glyphs live in a per-`Ui` generational arena; a text widget holds
+one compact handle into it, while the arena and its shaping state are shared by
+one `Rc<RefCell<_>>`. Registering another family invalidates the live arena
+entries and dirties their active owner widgets, so registration is also valid
+after the first shape. ai-app owns the `ai-app-icons` family, its Nerd Fonts
+subset, its codepoints, its license and the script that rebuilds it; the CSS
+generic names `sans-serif` and `monospace` continue to resolve through the
+platform.
 
 `cargo-iris` is an installable Cargo subcommand, rather than a script callers
 must find inside an Iris checkout. `cargo iris apk` builds the Rust `cdylib`