iris is the framework alone; the app is one crate in app-rust/

Iris: "the organization of the rust rewrite is a mess right now... there
shouldn't be anything related to the app inside of iris. Iris is supposed
to be the UI framework alone." And, on the crate count: "I'm confused why
the app only code needs more than one crate though."

Nine cargo workspaces become three, and the port's project code -- which
sat in five places, four of them inside the framework -- becomes one crate,
`ai-app`, in `app-rust/`:

  client-core                -> app-rust/src/client
  iris/transcript-ui         -> app-rust/src/ui
  iris/transcript-fixture    -> app-rust/src/ui/fixture.rs + tests/ + touch/
  iris/desktop-app           -> app-rust/src/desktop + src/bin_desktop.rs
  iris/android-app           -> app-rust/src/android + android-project/
  android-shell              -> app-rust/src/shell

iris/ keeps core, macro, the iris crate, tabs-ui and rig-input, and now
mentions no session, transcript, setup or server anywhere.

Only two of the old splits had a reason that survived reading. event-model
stays a crate at the repo root because server/ depends on it too, so a
crate is what makes the backend and the app agree by construction. The two
Android .so names looked like a hard constraint -- a package produces one
library artifact -- until P2 turned out to already plan merging those two
Android apps into one; both faces now come out of libai_app.so, picked
apart by features so `--no-default-features --features shell` keeps wgpu,
parley and iris out of the Compose app's APK. docs/RUST.md's "One app
crate" has the rest, including what each remaining feature is for.

DECISIONS.md and SUBAGENTS.md move into docs/ with everything else.

Verified: ./run-tests.sh and `cd iris && cargo test` green, clippy and fmt
clean in all five workspaces, `cargo ndk -t x86_64` links libai_app.so,
build-apk.sh produces an APK that installs and launches on this checkout's
emulator (Gl ... virgl, as expected), and the phone-sized headless
screenshot renders the transcript unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/platform.rs

@@ -0,0 +1,22 @@
+//! Capabilities a widget tree needs from whatever is hosting it, that
+//! neither iris nor the app can perform itself.
+//!
+//! Same shape as [`crate::attr::FocusHost`], and for the same reason: the
+//! interface is declared here, below, and implemented by each backend
+//! above (`default/platform.rs`, `android/platform.rs`), so a widget can
+//! ask for the capability by trait bound instead of a caller threading a
+//! callback down through every builder.
+
+/// Hand a URL to whatever the platform opens URLs with.
+///
+/// One method rather than a general "run an intent"/"exec" surface: the
+/// only thing a transcript needs is to follow a link a reader tapped, and
+/// a narrower capability is a narrower thing to get wrong.
+///
+/// **Nothing is reported back.** There is no answer worth branching on --
+/// the platform either shows a browser or does not, and both are outside
+/// this process -- so failures are logged where they happen (each impl)
+/// rather than turned into a `Result` every call site would discard.
+pub trait OpenUrl {
+    fn open_url(&mut self, url: &str);
+}