Extract kiln-ui: standalone game-agnostic ratatui widget toolkit

Widgets (CodeEditor, TextField, Dialog) plus clipboard/text internals,
extracted from the kiln workspace. No game-engine dependency.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "kiln-ui"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+# text-only clipboard: drop the default `image-data` deps (image/tiff/jpeg), add Linux Wayland support
+arboard = { version = "3", default-features = false, features = ["wayland-data-control"] }
+# Pinned to 0.30.x: consumers (e.g. kiln-tui) exchange ratatui types with these widgets,
+# so their ratatui must unify with this one.
+ratatui = "0.30.1"
+syntect = "5.3.0"
+unicode-width = "0.2"