fix: eliminate 3 migration pitfalls in xterm-zerolag-input

Pitfall 3 (REAL BUG): setFlushed() called _render() unconditionally.
During tab switch, rendering against a stale buffer found the old
session's prompt and locked the column position via the flushed-text
column lock. After the new buffer loaded, rerender() kept the stale
column. Fix: add render parameter (default true), pass false during
tab-switch restore to defer rendering until buffer is loaded.

Pitfall 2: Tab completion "undo" required calling clearFlushed() +
resetBufferDetection() separately — easy to get wrong. Add
undoDetection() convenience method that atomically clears flushed
state and re-enables detection.

Pitfall 1: Backspace Map sync is app-specific (can't be fixed in lib),
but removeChar() return value + getFlushed() make the pattern clean.
Updated tab-switch test to demonstrate the correct pattern.

Tests: 78 total (+4 new)
- setFlushed render=false prevents stale column lock
- setFlushed render=false keeps overlay hidden
- undoDetection clears flushed + re-enables detection
- undoDetection preserves pending text

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/xterm-zerolag-input/src/zerolag-input-addon.ts

@@ -271,11 +271,16 @@ export class ZerolagInputAddon implements XtermAddon {
      *
      * @param count - Number of characters flushed
      * @param text - The actual flushed text (avoids reading stale terminal buffer)
+     * @param render - Whether to re-render immediately (default: `true`).
+     *   Pass `false` when restoring flushed state during a tab/session switch
+     *   before the new buffer has loaded — rendering against a stale buffer
+     *   would lock the prompt column to the wrong position. Call `rerender()`
+     *   explicitly after the buffer finishes loading.
      */
-    setFlushed(count: number, text: string): void {
+    setFlushed(count: number, text: string, render = true): void {
         this._flushedOffset = count;
         this._flushedText = text;
-        this._render();
+        if (render) this._render();
     }
 
     /**
@@ -347,6 +352,20 @@ export class ZerolagInputAddon implements XtermAddon {
         this._bufferDetectDone = false;
     }
 
+    /**
+     * Undo the last `detectBufferText()` call — clears flushed state and
+     * re-enables detection.
+     *
+     * Use case: Tab completion detection found text that matches the
+     * pre-tab baseline (no real completion happened). Call this to undo
+     * the detection so it can retry on the next flush cycle.
+     */
+    undoDetection(): void {
+        this._flushedOffset = 0;
+        this._flushedText = '';
+        this._bufferDetectDone = false;
+    }
+
     /**
      * Suppress buffer detection until the next `clear()` or
      * `resetBufferDetection()` call.