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>

.gitignore

@@ -21,3 +21,6 @@ certs/
 config.ron
 config.json
 sessions/
+
+# iris, the in-house UI library, is vendored at iris/ and built by cargo.
+iris/target/