Vendor iris, the in-house UI library, at iris/

Iris's decision: it lives in this repository for now, included by path,
with its history left in the iris/iris repository on the gitea remote
(this is its main at 7b54aaf, byte-identical to the public GitHub copy).
It gets its own repository back once it has proved itself here.

RUST.md's I0 records the decision and what the first build said: the
tree does not compile on the current nightly because const_trait_impl
now requires traits to be declared 'const trait', which is the first
item of I0b.

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

RUST.md

@@ -447,19 +447,40 @@ accepted.
 ### The iris track
 
 These build iris up to carry the app. Each is a feature added to iris
-with a pass condition, in dependency order. Work in iris's own repository,
-pinned here by commit, and record in this file what each step measured.
+with a pass condition, in dependency order. Work in `iris/` in this
+repository on the `rustify` branch, and record in this file what each
+step measured.
 
-- [ ] **I0 — bring iris in-house.** Decide where it lives — the
-      recommendation is a repository of its own on the gitea remote,
-      consumed here the way `wg-app-link` is (a submodule pinned to a
-      commit), so the app and the library version-lock and the library
-      stays usable by the desktop app on its own. Pin a dated nightly in
-      `rust-toolchain.toml`; list every `#![feature]` gate with what it is
-      for, so each can be retired when it stabilises. Get `cargo build`,
-      `clippy` and `fmt` clean at the defaults and the `tabs` example
-      running in a window. Pass: a fresh clone builds unattended from the
-      pinned toolchain.
+- [x] **I0a — where iris lives (decided 2026-09-04).** For now it is
+      **vendored at `iris/` in this repository**, history not carried,
+      and consumed by path. Iris's decision: keep it close while it is
+      being reshaped for this app, and give it back its own repository —
+      `iris/iris` on the gitea remote, which already holds the full
+      244-commit history, on a branch of its own — once it has proved
+      itself. The vendored tree is that repository's `main` at
+      `7b54aaf` ("readme", 2026-01-29), byte-identical to the public
+      GitHub copy, so a later reconciliation has a known base. A crate
+      that uses it says `iris = { path = "../iris" }`.
+- [ ] **I0b — make it build here.** Pin a dated nightly in
+      `iris/rust-toolchain.toml` (the machine has
+      `nightly-x86_64-unknown-linux-gnu`, rustc 1.100.0-nightly
+      2026-08-25); list every `#![feature]` gate with what it is for, so
+      each can be retired when it stabilises. Get `cargo build`, `clippy`
+      and `fmt` clean at the defaults and the `tabs` example running in a
+      window (this VM has no display: a headless compositor is what
+      `emu` uses for the emulator, and the same trick serves here).
+      Pass: a fresh clone builds unattended from the pinned toolchain.
+      **Measured 2026-09-04**: `cargo +nightly check` on that nightly
+      fails in `iris-core` with 36 errors, all one cause — the
+      `const_trait_impl` feature changed shape between January and
+      August: `impl const Trait for T` is now rejected with "expected a
+      trait, found type" unless the trait itself is declared
+      `const trait`, at seven sites (`num.rs`, `align.rs`, `axis.rs`,
+      `pos.rs`, `color.rs`, `math.rs`, `vec2.rs`), and the unresolved
+      `UiVec2`/`Vec2`/`impl_op` imports cascade from those. That is the
+      nightly-drift risk in one build; the fix is mechanical (declare
+      the traits `const trait`), and it is the first item of I0b rather
+      than pinning back to a January nightly, which would only defer it.
 - [ ] **I1 — the text stack decision.** iris uses cosmic-text; the
       transcript needs rich inline spans (links, code chips, colour),
       selection across many widgets with the platform's handles on the