fix(terminal): let a composition-only overlay follow the prompt, repaint it on removeChar, document the API (#499 review)

Merge-time fixes for the three findings of the third review round of #499.

- minor: a composition on an empty prompt did not follow the prompt after
  output or a resize. The post-write re-place in flushPendingWrites and the
  resize observer both ran rerender() only when hasPending was true, and
  hasPending deliberately excludes the composition, so the first word of a
  prompt (an overlay holding only a composition) stayed on the old row over
  whatever output moved there. Both sites now call rerender() unconditionally;
  it already returns early when there is nothing to draw, so nothing changes
  without a composition. New browser case drives the real
  batchTerminalWrite/flushPendingWrites path against real xterm 6 and the
  overlay built from source, moves the prompt from row 0 to row 3 and checks
  the overlay follows (it fails on the old guard, overlay left on row 0), with
  a parity case for pending text. The structure test pins the post-write site
  through vm and the resize site, which is a closure inside initTerminal(), by
  source.
- nit: removeChar() dropped the composition but did not repaint on its false
  path, leaving a composition-only overlay on screen showing text the addon no
  longer held. It now hides the overlay there when a composition was dropped.
  Package tests cover that path and the flushed path repainting without the
  tail.
- nit: the package README did not document setComposition() or the
  composition getter and described hasPending as "any content". Added both to
  the API tables plus a short IME composition section, reworded hasPending
  (pending or flushed text, excludes the composition), and made the quick
  start re-render unconditionally instead of teaching the hasPending guard.
  The hasPending JSDoc says the same.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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

@@ -208,9 +208,13 @@ export class ZerolagInputAddon implements XtermAddon {
    * - `'flushed'`: A character was removed from text already sent to the PTY.
    *   The consumer SHOULD send backspace to the PTY.
    * - `false`: Nothing to remove. The consumer should NOT send backspace.
+   *
+   * Any IME composition is dropped in every case, and the overlay is repainted
+   * without it (hidden when nothing else is left).
    */
   removeChar(): 'pending' | 'flushed' | false {
     // A backspace that reaches the overlay means no composition is open.
+    const droppedComposition = this._composition.length > 0;
     this._composition = '';
     if (this._pendingText.length > 0) {
       this._pendingText = this._pendingText.slice(0, -1);
@@ -247,6 +251,9 @@ export class ZerolagInputAddon implements XtermAddon {
       return 'flushed';
     }
 
+    // Nothing to remove, but a composition-only overlay is still on screen
+    // drawing the text dropped above.
+    if (droppedComposition) this._hide();
     return false;
   }
 
@@ -460,7 +467,13 @@ export class ZerolagInputAddon implements XtermAddon {
     return this._pendingText;
   }
 
-  /** Whether there is any overlay content (pending or flushed). */
+  /**
+   * Whether there is pending or flushed text. Excludes the IME composition,
+   * which is never sent, so an overlay showing only a composition reports
+   * `false` while still on screen. To re-place the overlay after output or a
+   * resize, call `rerender()` unconditionally: it is a no-op when there is
+   * nothing to draw.
+   */
   get hasPending(): boolean {
     return this._pendingText.length > 0 || this._flushedOffset > 0;
   }