Make iris build: pin a dated nightly and migrate const-trait impls

The vendored January tree did not parse at all on a current nightly: 36
errors in iris-core, all from one syntax change. `impl const Trait for T`
is now `const impl Trait for T`, with generics on the `impl`. Bounds are
unaffected, and the traits were already declared `const trait` -- so the
diagnosis recorded in RUST.md was wrong, and pinning back to a January
nightly would only have deferred this. Everything else (the unresolved
UiVec2/Vec2/impl_op imports, a Color<u8> resolving to wgpu_types::Color)
cascaded from the seven files that failed to parse.

The pin is dated rather than `nightly` because that is exactly the
failure: a rolling channel moving under a build Dev Updater runs
unattended. It carries the components and Android targets too, so a
fresh clone provisions itself.

Also drops two `#![feature]` gates the compiler reports as declared and
unused, since the build stays warning-clean, and takes rustfmt's import
order in attr.rs.

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

iris/core/src/attr.rs

@@ -1,4 +1,4 @@
-use crate::{UiRsc, WidgetIdFn, WidgetLike, WeakWidget};
+use crate::{UiRsc, WeakWidget, WidgetIdFn, WidgetLike};
 
 pub trait WidgetAttr<Rsc, W: ?Sized> {
     type Input;