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>

iris/src/widget/position/offset.rs

@@ -0,0 +1,21 @@
+use crate::prelude::*;
+
+pub struct Offset {
+    pub inner: StrongWidget,
+    pub amt: UiVec2,
+}
+
+impl Widget for Offset {
+    fn draw(&mut self, painter: &mut Painter) {
+        let region = UiRegion::FULL.offset(self.amt);
+        painter.widget_within(&self.inner, region);
+    }
+
+    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len {
+        ctx.width(&self.inner)
+    }
+
+    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len {
+        ctx.height(&self.inner)
+    }
+}