feat(kebab-tui): P9-1 Ratatui shell + Library pane

새 crate `kebab-tui` 가 §8 facade rule 따라 `kebab-app` 만 import.
Ratatui 0.28 + crossterm 0.28 기반 shell 이 다음을 제공:

- `App` 구조체: config + focus + library + 3 Option sub-state slot
  (search/ask/inspect — p9-2/3/4 가 자기 모듈에서 채우는 parallel-safety
  contract). p9-1 외에 App 정의 손대지 않음.
- `Pane` enum (Library/Search/Ask/Inspect/Jobs).
- `KeyOutcome` (Continue/Quit/SwitchPane/Refresh).
- `LibraryState` + 내부 inner: docs / list_state / filter / filter_edit /
  needs_refresh / loading / pending_g.
- `render_library` (Frame, area, &App) — heading/body, filter overlay
  toggleable, Korean/wide-char 너비는 unicode-width 로 계산.
- `handle_key_library`: j/k/Down/Up 이동, gg/G 끝, f 필터 overlay,
  /=>Search ?=>Ask Enter=>Inspect, q/Esc 종료. error overlay 가 켜
  있으면 어떤 키든 dismiss.
- 필터 overlay: tags_any (CSV) + lang 두 필드, Tab cycle, Enter
  apply→Refresh, Esc cancel.
- `ErrorOverlay`: anyhow chain 캡쳐 후 popup 렌더 (Clear + 빨간 border).
- 터미널 lifecycle: `TuiTerminal` 가 enter raw mode + alt screen,
  Drop 이 종료 시 (panic 포함) restore — 사용자 쉘 깨지지 않게.
- 비동기 없음: facade 호출은 main thread 동기. v1 의 brief hang 수용.

CLI: `kebab tui` 서브커맨드 추가, --config 받아 App::new + run.

테스트 10건 (`tests/library.rs`, TestBackend 사용):
- 빈 library / 3-doc render / q,Esc quit / / Search 전환 / ? Ask 전환
- Enter 빈 list 무동작 / Enter Inspect 전환 / j 이동 (3-step clamp) /
  f 필터 overlay → 입력 → Enter Refresh.

Test seam: `App::populate_library_for_testing` (#[doc(hidden)]) 가
`pub(crate)` inner 를 우회. spec parallel-safety contract 그대로 유지.

Spec deviation (HOTFIXES `2026-05-02 P9-1`):
- `render_library` 의 `<B: Backend>` generic 제거 — ratatui 0.28 의 Frame
  이 backend-agnostic.
- `populate_library_for_testing` 추가 (test seam, 공식 API 아님).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tasks/HOTFIXES.md

@@ -14,6 +14,21 @@ historical contract that was implemented; this file accumulates the
 deltas so phase 5+ readers can find the live behavior without diffing
 git history.
 
+## 2026-05-02 — P9-1 TUI Library: render_library generic + test seam
+
+**Discovered**: P9-1 implementation start.
+
+**Symptom 1 (cosmetic)**: `tasks/p9/p9-1-tui-library.md` § Public surface declares `pub fn render_library<B: ratatui::backend::Backend>(f: &mut ratatui::Frame, area: Rect, state: &App)`. ratatui 0.28 dropped the backend generic from `Frame` (it's bound at `Terminal` initialisation, not at the render call site). The `<B: Backend>` parameter would be unused on the function and clippy `-D warnings` rejects unused generic parameters.
+
+**Fix 1**: `render_library(f: &mut Frame, area: Rect, state: &App)` — no generic parameter. The function still works against any backend the `Terminal` was opened with (CrosstermBackend in production, TestBackend in snapshot tests). No call-site impact.
+
+**Symptom 2 (test seam)**: `LibraryState.inner` is `pub(crate)` per the spec's parallel-safety contract — p9-2/3/4 must not mutate `LibraryState` directly. Snapshot tests in `tests/library.rs` (an integration test, NOT a unit test in the same module) cannot reach `pub(crate)` fields, so they cannot inject docs without going through `kebab-app::list_docs_with_config` (which would stand up a TempDir SQLite KB just to populate three rows).
+
+**Fix 2**: new `App::populate_library_for_testing(&mut self, Vec<DocSummary>)` marked `#[doc(hidden)]`. Lets snapshot tests inject docs hermetically while keeping the parallel-safety boundary intact for normal callers (the helper is officially "test seam, not part of the UI API"). Same shape as `kebab-app::*_with_config` test seams from P3-5.
+
+**Amends**:
+- tasks/p9/p9-1-tui-library.md (`render_library` no longer generic; `populate_library_for_testing` test seam added).
+
 ## 2026-05-02 — P7-3 PDF ingest wiring: chunker_version deviation + storage UNIQUE bug
 
 **Discovered**: P7-3 implementation start.