fix(terminal): clear Ink UI on REPL shutdown

## Bug 详情
使用 /exit 退出交互式 REPL 后，终端可能保留提示输入、命令补全或其他 Ink 临时 UI，导致 shell prompt 与旧 UI 重叠，出现光标和显示异常。

## 根因
shutdown 路径会恢复 raw mode、bracketed paste、focus reporting 等终端模式，但普通主屏幕 REPL 不会清理 Ink 当前占用的可见区域。进程退出前 React 也不一定能完成 /exit 输入清空后的重新渲染。

## 修复方案
在 Ink 实例中增加主屏幕 shutdown 清理方法，根据最后一帧和声明光标位置清除 Ink 占用区域；gracefulShutdown 在非 alternate-screen 场景调用该方法，alternate-screen 继续保留原有退出逻辑。

## 变更要点
- 新增 ERASE_TO_END_SCREEN CSI 常量用于从当前光标位置清到屏幕底部
- 增加 Ink.clearMainScreenForShutdown() 统一计算并清理主屏幕 REPL 区域
- 在 gracefulShutdown 的终端清理阶段区分 alternate-screen 和主屏幕 Ink 实例

## 自测
- 已运行局部 Biome 检查：bunx biome check packages/@ant/ink/src/core/ink.tsx packages/@ant/ink/src/core/termio/csi.ts src/utils/gracefulShutdown.ts
- 已通过 Bun 直接导入验证新增 Ink 方法和 gracefulShutdown 模块可解析
- 全量 typecheck/build 当前被既有无关问题阻断：ctx_viz/LogoV2/doubaoSTT 类型错误，以及 generate-review-builtin 访问 git@github.com:zgsm-ai/costrict-review.git 失败

packages/@ant/ink/src/core/ink.tsx

@@ -68,6 +68,7 @@ import {
 import { SYNC_OUTPUT_SUPPORTED, supportsExtendedKeys, type Terminal, writeDiffToTerminal } from './terminal.js';
 import {
   CURSOR_HOME,
+  CURSOR_LEFT,
   cursorMove,
   cursorPosition,
   DISABLE_KITTY_KEYBOARD,
@@ -75,6 +76,7 @@ import {
   ENABLE_KITTY_KEYBOARD,
   ENABLE_MODIFY_OTHER_KEYS,
   ERASE_SCREEN,
+  ERASE_TO_END_SCREEN,
 } from './termio/csi.js';
 import {
   DBP,
@@ -1054,6 +1056,20 @@ export default class Ink {
     }
   }
 
+  /**
+   * Clear the visible main-screen region currently owned by Ink.
+   * Used by graceful shutdown for interactive REPL exits, where transient UI
+   * should not remain underneath the shell prompt.
+   */
+  clearMainScreenForShutdown(): void {
+    if (!this.options.stdout.isTTY || this.altScreenActive) return;
+
+    const cursor = this.displayCursor ?? this.frontFrame.cursor;
+    const topOffset = Math.min(Math.max(cursor.y, 0), Math.max(this.frontFrame.viewport.height - 1, 0));
+    const clearFromTop = cursorMove(0, -topOffset) + CURSOR_LEFT + ERASE_TO_END_SCREEN;
+    writeSync(1, clearFromTop);
+  }
+
   /** @see drainStdin */
   drainStdin(): void {
     drainStdin(this.options.stdin);

packages/@ant/ink/src/core/termio/csi.ts

@@ -216,6 +216,9 @@ export function eraseToEndOfScreen(): string {
   return csi('J')
 }
 
+/** Erase from cursor to end of screen - constant form */
+export const ERASE_TO_END_SCREEN = csi('J')
+
 /** Erase from cursor to start of screen (CSI 1 J) */
 export function eraseToStartOfScreen(): string {
   return csi(1, 'J')

src/utils/gracefulShutdown.ts

@@ -65,23 +65,23 @@ function cleanupTerminalModes(): void {
     // we're busy unmounting. Otherwise events arrive during cooked-mode
     // cleanup and either echo to the screen or leak to the shell.
     writeSync(1, DISABLE_MOUSE_TRACKING)
-    // Exit alt screen FIRST so printResumeHint() (and all sequences below)
-    // land on the main buffer.
-    //
-    // Unmount Ink directly rather than writing EXIT_ALT_SCREEN ourselves.
-    // Ink registered its unmount with signal-exit, so it will otherwise run
-    // AGAIN inside forceExit() → process.exit(). Two problems with letting
-    // that happen:
-    //   1. If we write 1049l here and unmount writes it again later, the
-    //      second one triggers another DECRC — the cursor jumps back over
-    //      the resume hint and the shell prompt lands on the wrong line.
-    //   2. unmount()'s onRender() must run with altScreenActive=true (alt-
-    //      screen cursor math) AND on the alt buffer. Exiting alt-screen
-    //      here first makes onRender() scribble a REPL frame onto main.
-    // Calling unmount() now does the final render on the alt buffer,
-    // unsubscribes from signal-exit, and writes 1049l exactly once.
     const inst = instances.get(process.stdout)
     if (inst?.isAltScreenActive) {
+      // Exit alt screen FIRST so printResumeHint() (and all sequences below)
+      // land on the main buffer.
+      //
+      // Unmount Ink directly rather than writing EXIT_ALT_SCREEN ourselves.
+      // Ink registered its unmount with signal-exit, so it will otherwise run
+      // AGAIN inside forceExit() -> process.exit(). Two problems with letting
+      // that happen:
+      //   1. If we write 1049l here and unmount writes it again later, the
+      //      second one triggers another DECRC — the cursor jumps back over
+      //      the resume hint and the shell prompt lands on the wrong line.
+      //   2. unmount()'s onRender() must run with altScreenActive=true (alt-
+      //      screen cursor math) AND on the alt buffer. Exiting alt-screen
+      //      here first makes onRender() scribble a REPL frame onto main.
+      // Calling unmount() now does the final render on the alt buffer,
+      // unsubscribes from signal-exit, and writes 1049l exactly once.
       try {
         inst.unmount()
       } catch {
@@ -89,6 +89,8 @@ function cleanupTerminalModes(): void {
         // so printResumeHint still hits the main buffer.
         writeSync(1, EXIT_ALT_SCREEN)
       }
+    } else if (inst) {
+      inst.clearMainScreenForShutdown()
     }
     // Catches events that arrived during the unmount tree-walk.
     // detachForShutdown() below also drains.
@@ -169,9 +171,7 @@ function printResumeHint(): void {
 
       writeSync(
         1,
-        chalk.dim(
-          `\nResume this session with:\ncsc --resume ${resumeArg}\n`,
-        ),
+        chalk.dim(`\nResume this session with:\ncsc --resume ${resumeArg}\n`),
       )
       resumeHintPrinted = true
     } catch {