RUST.md: record E0 and I0b, with what they measured

E0 is done (NDK r29, cargo-ndk 4.1.2, verified by cross-compiling to both
ABIs) and I0b is done. Corrects this file's guess at why iris would not
build, notes the const-traits family as the gates to re-read whenever the
pin is advanced, and records the cold build weight against the "slow in
debug" worry: 43s and 2.1 GB plain, 1m46s and 1.5 GB with dependencies at
opt-level 2.

Also records an open defect found on the way -- iris sometimes keeps its
pre-configure window size for good -- with what was ruled out, since it
is timing-sensitive enough that any added print hides it, and I2 will
meet it on every rotation.

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

RUST.md

@@ -418,9 +418,20 @@ Android backend wants API 26 (a libc symbol). The real phone has Vulkan.
 Per the standing rule, a rig limit is something to fix before it is
 accepted.
 
-- [ ] **E0 — toolchain.** No NDK is installed (`~/Android/Sdk/ndk` is
-      empty; the Android Rust targets are). Install the NDK under the
-      user-owned SDK, `cargo-ndk`, and record the versions here.
+- [x] **E0 — toolchain (done 2026-09-04).** Installed under the
+      user-owned SDK: **NDK r29 (`29.0.14206865`)**, 2.4 GB at
+      `~/Android/Sdk/ndk/29.0.14206865`, the newest stable — r30 is still
+      at rc.3. **cargo-ndk 4.1.2**. Verified by cross-compiling a scratch
+      `cdylib` to both ABIs: `file` reports "for Android 26, built by NDK
+      r29 (14206865)" for `aarch64-linux-android` and
+      `x86_64-linux-android`. Two things to know at the call site.
+      **cargo-ndk 4's API-level flag is `-P`, not `-p`** — `-p` is now
+      passed through to cargo as `--package`, so the old
+      `cargo ndk -t arm64-v8a -p 26` panics with `unknown package: 26`
+      *and dumps the whole environment to stdout* as a bug report, which is
+      worth not doing in a log somebody might paste. And the Android
+      targets were installed for **stable** only; the pinned nightly needs
+      its own, which `iris/rust-toolchain.toml` now declares.
 - [ ] **E1 — android-view's Masonry demo on this emulator.** Pass: it
       builds with `cargo-ndk` + Gradle, renders on the GPU, and the phone's
       own keyboard types into its editor with autocorrect and suggestions.
@@ -461,26 +472,111 @@ step measured.
       `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.
+- [x] **I0b — make it build here (done 2026-09-04).** iris now builds,
+      clippy-clean and rustfmt-clean at the defaults, on a pinned dated
+      nightly, and the `tabs` example draws on this VM's GPU.
+
+      **The pin** is `nightly-2026-09-03` (rustc 1.100.0-nightly,
+      `2e2b193f8`), declared in `iris/rust-toolchain.toml` along with the
+      `clippy`/`rustfmt` components and the two Android targets, so a
+      fresh clone provisions itself. It is dated rather than `nightly`
+      because the whole failure below was a rolling channel moving under
+      an unattended build. Installed with `--profile minimal`: 912 MB.
+
+      **The 36 errors were one syntax change, and the earlier diagnosis in
+      this file was wrong.** It is not that a trait must now be declared
+      `const trait` — the vendored tree already declares them that way,
+      which is how it was written in January. What changed is the *impl*
+      keyword order: `impl const Trait for T` is now
+      `const impl Trait for T`, and generics go on the `impl`
+      (`const impl<T: [const] Foo> Bar for T`). Bounds are unaffected;
+      `T: const Foo`, `T: [const] Foo` and `impl const Foo` in argument
+      position all still compile. Everything else — the unresolved
+      `UiVec2`/`Vec2`/`impl_op` imports, and a `Color<u8>` that resolved
+      to `wgpu_types::Color` — cascaded from the seven files that failed
+      to parse. The rewrite was mechanical across 20 sites and took the
+      workspace from 36 errors to 0.
+
+      **`#![feature]` gates, 12 after this step** (two were declared and
+      unused, and were removed: `map_try_insert`, `const_cmp`).
+      Load-bearing and worth watching: `const_trait_impl`, `const_ops`,
+      `const_convert`, `const_destruct` are the const-traits family and
+      the one that has already broken once — they move together, so
+      advancing the pin means re-reading this section. `unboxed_closures`
+      + `fn_traits` (postfix builder API) and `unsize` +
+      `coerce_unsized` (widget handles) are pairs. The rest are
+      individually small: `macro_metavar_expr_concat`, `portable_simd`,
+      `associated_type_defaults`, `option_into_flat_iter`, and `gen_blocks`
+      in the top crate.
+
+      **Running it headless.** `iris/run-headless.sh EXAMPLE [--shot PNG]`
+      with `iris/headless.conf`, the same trick `emu` uses: a headless
+      sway, and `grim` for the picture. It deliberately starts its *own*
+      compositor rather than joining `emu`'s — sway tiles, so adding a
+      window to the one an emulator sits in resizes that emulator.
+      Unlike `emu`'s it disables Xwayland, since winit speaks Wayland.
+      **This VM has a real GPU for this**: Vulkan 1.4 through Venus onto
+      the host's RX 7900 XT, and GL 4.6 through virgl — so desktop wgpu
+      work here is not software-rasterised, unlike inside the emulator.
+
+      **iris has no tests at all** (`cargo test --workspace`: 0 passed
+      across 6 targets). Nothing to keep passing, and nothing to catch a
+      regression — worth knowing before I1 changes the text stack.
+
+      **`iris-core` no longer depends on winit, and now cross-compiles to
+      Android.** It wanted exactly one thing from it — `PhysicalSize<u32>`
+      in `UiRenderNode::resize`'s signature, for two numbers it immediately
+      turned into floats — and that pulled a whole windowing backend into
+      the layer below it, the wrong direction. `resize` takes
+      `impl Into<Vec2>` now, like `UiRenderState::resize` beside it already
+      did. The consequence is the point: with winit in the graph an Android
+      build of the core failed in `android-activity` (which needs a backend
+      feature nothing here selects), and without it
+      `cargo ndk -t arm64-v8a -P 26 build -p iris-core` finishes in 30s and
+      produces an rlib, wgpu's Android backend included. So **iris's
+      widget, layout and render core already builds for the phone**, and
+      what I2 has to supply is the surface, the input and the IME — not a
+      port of the library.
+
+      **Build weight, cold, on this VM's 8 cores** (`rm -rf target`, then
+      `cargo build --example tabs`), since "the Linebender stack is slow in
+      debug" was the worry behind this question: plain debug **43s** and a
+      2.1 GB `target/`; with the `[profile.dev.package."*"] opt-level = 2`
+      knob, **1m46s** and 1.5 GB. So iris's own wgpu + winit + cosmic-text
+      graph is not the slow thing — which makes it a calibration for E1
+      rather than an answer about Masonry, whose graph adds Vello, Parley,
+      Fontique and Skrifa. Runtime cost of the knob was not measured here.
+
+      **Open defect found while doing this: iris sometimes never adopts
+      the window's real size.** Measured on the headless rig, ~3 starts in
+      15: the `tabs` example settles showing its 800×600 startup layout in
+      the top-left of a 1920×1200 surface, black around it, and stays that
+      way indefinitely — it is not a screenshot taken too early, since the
+      picture is byte-identical for the next four seconds. What is *not*
+      the cause, each checked: the winit event order is identical in good
+      and bad runs (`Resized(800×600)`, two redraws, `Resized(1920×1200)`,
+      one redraw), the swapchain reports `1920×1200` and
+      `suboptimal=false` on that last draw, and `output_size` is
+      `(1920, 1200)` going into it. It is timing-sensitive in the way that
+      makes it expensive: adding a single `eprintln!` anywhere in the draw
+      or event path hides it completely (0 in 16), which is why the
+      instrumentation above could not catch it in the act. A pointer move
+      does not repair it, because iris only redraws when something
+      changed; an output mode change does, because that is another resize.
+      Left open rather than guessed at. It matters most for **I2**, where
+      every rotation and every keyboard open is a resize, so a stale frame
+      would be the normal case rather than a rare one; a Wayland-level
+      trace of the xdg-surface configure/ack/commit sequence is the next
+      step, not more `eprintln`.
+
+      One thing was fixed on the way, and it is not that bug: `update`
+      redrew everything when `resized` was set, but `needs_redraw` — which
+      is what decides whether to *ask* for a frame — did not know about
+      `resized` at all. The two now share one `needs_redraw_all`, since a
+      condition in one and not the other is a frame nobody requests. It is
+      latent on Wayland only because winit asks for a redraw after a resize
+      by itself; on Android, where the surface work of I2 will not have
+      winit underneath it, nothing else here would have asked.
 - [ ] **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