Add RUST.md: the plan for moving the app to Rust

Research and measurements from 2026-09-04: what the Compose app has to
reproduce, why Android text input and rich selectable text decide the
framework, the options considered (Masonry as the yardstick, iris as the
in-house library to build up; Slint, iced, egui, Makepad rejected with
reasons), how thin the Java shell can be, building the APK without
Gradle, and the ordered experiments with pass conditions. Includes the
emulator Vulkan findings: Venus is blocked by this emulator's gfxstream,
SwiftShader over the emulator's own ICD works.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

AGENTS.md

@@ -46,6 +46,10 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   before touching `TranscriptCache.kt`, `TranscriptSource.kt`, or the opening
   and stream effects in `SessionScreen.kt`.
 - `TODO.md` — the working list.
+- `RUST.md` — the plan for moving the app to Rust (on the `rustify`
+  branch of the `ai-app-2` clone): what has to be reproduced, the
+  framework decision, and the ordered experiments with their pass
+  conditions. Read it before touching anything under that branch.
 - `.dev-updater.ron` — what Dev Updater builds here: the server (run as
   `service: Managed(…)`, supervised by Dev Updater's own implementation
   rather than a script kept here) and the APK, in parallel. It points at