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>

core/src/primitive/layer.rs

@@ -1,9 +1,6 @@
 use std::ops::{Index, IndexMut};
 
-use crate::{
-    render::{MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, Primitives},
-    util::to_mut,
-};
+use crate::{render::LayerOrder, util::to_mut};
 
 pub type LayerId = usize;
 
@@ -39,7 +36,10 @@ struct Child {
     tail: usize,
 }
 
-pub type PrimitiveLayers = Layers<Primitives>;
+/// The draw order of every layer. The primitives themselves live in one
+/// arena beside this (`UiRenderState::primitives`); a layer names the
+/// slots it draws, which is what its vertex buffer is.
+pub type PrimitiveLayers = Layers<LayerOrder>;
 
 impl<T: Default> Layers<T> {
     pub fn new() -> Layers<T> {
@@ -119,20 +119,6 @@ impl<T: Default> Layers<T> {
     }
 }
 
-impl PrimitiveLayers {
-    pub fn write<P: Primitive>(
-        &mut self,
-        layer: LayerId,
-        info: PrimitiveInst<P>,
-    ) -> PrimitiveHandle {
-        self[layer].write(layer, info)
-    }
-
-    pub fn free(&mut self, h: &PrimitiveHandle) -> MaskIdx {
-        self[h.layer].free(h)
-    }
-}
-
 impl<T: Default> Default for Layers<T> {
     fn default() -> Self {
         Self::new()