fix(terminal): address review of the iOS IME preview

- Observe keydown in the capture phase on terminal.element, an ancestor of
  the helper textarea, so the controller sees it before xterm's own capture
  listener finalizes the composition and emits the commit through onData.
  Finalize on exactly the keys CompositionHelper.keydown does (every keyCode
  except 20/229/16/17/18), ignoring isComposing and key as xterm does.
- Bound awaitingCommit with the same 2 s fallback as the committed phase, so
  a composition whose commit never reaches onData cannot turn the next
  unrelated keystroke or paste into an IME commit.
- pagehide resets the controller instead of destroying it, so a back-forward
  cache restore keeps the preview working.
- Give the preview an opaque background from the terminal theme.
- Route an IME commit through the ordinary printable/paste local echo branch
  and complete the commit afterwards; drop the send-on-throw fallback.
- Pin the event order with an xterm stand-in registered in the capture phase
  ahead of the controller, and against real xterm in a browser test.
- CLAUDE.md: note the IME commit routing and the z-index 6 preview layer.

config/test-suites.ts

@@ -33,6 +33,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/split-pane-terminal.browser.test.ts',
   'test/split-pane-orchestration.browser.test.ts',
   'test/split-pane-auto-collapse.browser.test.ts',
+  'test/mobile-ime-preview.browser.test.ts',
 ];
 
 /**