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>

src/web/public/terminal-ui.js

@@ -1407,9 +1407,10 @@ Object.assign(CodemanApp.prototype, {
         // has to re-resolve their gate before the redraw, not just move them.
         this.applyLineageLineSettings?.();
         this.updateConnectionLines();
-        if (this._localEchoOverlay?.hasPending) {
-          this._localEchoOverlay.rerender();
-        }
+        // Unguarded on purpose: hasPending excludes an IME composition, so a
+        // composition-only overlay would stay on the old prompt row. rerender()
+        // is a no-op when the overlay has nothing to draw.
+        this._localEchoOverlay?.rerender();
         // Pane B (split view) has its own container and its own fit()/resize
         // frame — this observer only ever measured Pane A's container, so
         // without this call Pane B never learned about a window resize, an
@@ -4161,9 +4162,10 @@ Object.assign(CodemanApp.prototype, {
 
     // Re-position local echo overlay after terminal writes — Ink redraws can
     // move the ❯ prompt to a different row, making the overlay invisible.
-    if (this._localEchoOverlay?.hasPending) {
-      this._localEchoOverlay.rerender();
-    }
+    // Unguarded on purpose: hasPending excludes an IME composition, so a
+    // composition-only overlay (the first word of a prompt) would otherwise
+    // stay on the old row. rerender() is a no-op when there is nothing to draw.
+    this._localEchoOverlay?.rerender();
 
     // After Tab completion: detect the completed text in the overlay.
     // Use terminal.write('', callback) to defer detection until xterm.js