feat(kebab-tui): p9-fb-10 partial — CJK width helpers + render audit

`kebab-tui::input::{display_width, truncate_to_display_width}` 신규.
unicode-width 위에서 column-단위 width 계산 (ASCII=1, Hangul/CJK/
fullwidth=2, combining=0) + char-boundary 안전 truncate.

library.rs 의 중복 `truncate_to_display_width` private fn 제거 — 단일
source 로 통일. 9 unit tests + 1 integration render test (Korean +
Japanese fixture, TestBackend 80×20).

spec 의 `InputBuffer` struct 도입은 follow-up — Ask/Search/Editor pane
의 String + cursor 일괄 마이그레이션이 회귀 표면이 커서 helper 만
먼저 머지. backspace 는 모든 pane 이 이미 `String::pop()` 사용 → byte-
boundary 안전성 확보. crossterm 0.28 가 native IME composing 미노출 →
preedit handling out of scope.

spec status `planned` → `in_progress`. HOTFIXES.md 에 InputBuffer
struct deferral 사유 기록.

crates/kebab-tui/tests/library.rs

@@ -215,3 +215,45 @@ fn handle_key_library_f_opens_filter_overlay_then_enter_refreshes() {
     );
     assert_eq!(o2, KeyOutcome::Refresh);
 }
+
+/// p9-fb-10: Library renders Hangul / CJK titles without overflowing
+/// the title column. Smoke pin — render with a mixed Korean fixture
+/// and confirm no panic + the truncated width fits the column.
+#[test]
+fn library_renders_korean_titles_without_overflow() {
+    let docs = vec![
+        make_doc("ko/한글-노트.md", "러스트로 만드는 지식 베이스", vec!["rust", "한글"]),
+        make_doc("jp/漢字メモ.md", "日本語のテストドキュメント", vec!["jp"]),
+        make_doc("mix/hello-세계.md", "Hello, 세계 mixed title", vec!["mix"]),
+    ];
+    let app = app_with_docs(docs);
+    let backend = TestBackend::new(80, 20);
+    let mut terminal = Terminal::new(backend).unwrap();
+    terminal
+        .draw(|f| {
+            let area = Rect::new(0, 0, 80, 20);
+            render_library(f, area, &app);
+        })
+        .expect("render must not panic on CJK titles");
+    let buffer = terminal.backend().buffer().clone();
+    let rendered: String = (0..buffer.area.height)
+        .map(|y| {
+            (0..buffer.area.width)
+                .map(|x| buffer[(x, y)].symbol())
+                .collect::<String>()
+        })
+        .collect::<Vec<_>>()
+        .join("\n");
+    // At least one Hangul / Kanji glyph survives the render path.
+    // TestBackend renders wide chars one-per-cell with the trailing
+    // cell empty, so the joined string has spaces between adjacent
+    // wide chars — assert single glyphs, not multi-char substrings.
+    assert!(
+        rendered.contains('러') || rendered.contains('한'),
+        "expected a Hangul glyph in rendered frame: {rendered}"
+    );
+    assert!(
+        rendered.contains('日') || rendered.contains('漢'),
+        "expected a Kanji glyph in rendered frame: {rendered}"
+    );
+}