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>

README.md

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+# kiln-ui
+
+Reusable, game-agnostic terminal UI widgets built on [ratatui](https://ratatui.rs).
+
+Extracted from the [kiln](https://github.com/) project so the same widgets can drive
+any ratatui front-end (a game editor, a text editor, …). The crate has **no game-engine
+dependency**.
+
+## Widgets
+
+- **`code_editor::CodeEditor`** — a hand-written, modeless, keyboard-only full-screen
+  code editor: selection, undo/redo, word/paragraph motion, system clipboard, and
+  syntect syntax highlighting. (Highlighting is currently hardcoded to Rhai; a pluggable
+  syntax/theme is a planned follow-up.)
+- **`text_field::TextField`** — a single-line editable field (the building block for
+  dialog inputs).
+- **`dialog::Dialog<Ctx>`** — modal text-entry and filterable-list dialogs, generic over
+  a host context they mutate only through callbacks.
+- **`CursorOverlay` / `render_overlay` / `dim_area`** — the modal-overlay scaffolding:
+  a widget paints into a buffer and returns where the caret goes; the host places it.
+
+## Notes
+
+- **Not WASM-compatible**: pulls in `syntect` (highlighting) and `arboard` (system
+  clipboard). The ratatui core itself is WASM-friendly.
+- **ratatui version**: pinned to `0.30.x`. Consumers exchange ratatui types with these
+  widgets, so their `ratatui` must unify with this crate's.
+
+## Development
+
+```bash
+cargo build
+cargo test     # pure-logic unit tests, no TTY required
+```