fix(terminal): draw IME preview after local-echo text

With local echo on, committed text sits in the LocalEchoOverlay and does not
reach the PTY before Enter, so the PTY cursor that places the preview span
stays at the prompt start. The span's z-index 6 only counts inside
.xterm-helpers (its own z-index 5 stacking context), and the overlay is a
z-index 7 layer whose first line is opaque from the prompt column, so every
composition after the first one in a prompt was drawn under the overlay.

- xterm-zerolag-input: add setComposition(text) and a composition getter.
  The overlay draws the composition as an underlined, aria-hidden tail after
  its pending text, through the same wrapping and grow-upward layout. It is
  never part of pendingText, hasPending or anything sent; clear() and
  removeChar() drop it, and rerender()/refreshFont() keep it.
- terminal-ui.js: while local echo shows typed text (on, and not handed back
  to PTY echo by a nav key), render and clear the preview through
  setComposition. The helper span stays for local echo off, and as the
  fallback when the overlay cannot place the text (no prompt found).
- Browser test against real xterm 6, the overlay bundled from its source
  and styles.css: a second composition after pending text is the topmost
  element after that text, and the commit lands in the overlay once. Unit
  tests for setComposition in the package and for the routing in the
  structure test.
- CLAUDE.md and architecture-invariants: state the preview's effective layer.

test/mobile-ime-preview-structure.test.ts

@@ -280,6 +280,63 @@ describe('mobile IME preview lifecycle', () => {
     expect(helpers.classList.contains('codeman-ime-preview-owned')).toBe(false);
   });
 
+  // With local echo on, typed text sits in the overlay (z-index 7) and the PTY
+  // cursor that places the span stays at the prompt start, under that text. The
+  // overlay draws the composition instead. Real-xterm proof of the covering:
+  // test/mobile-ime-preview.browser.test.ts.
+  function withOverlay(app: App, visible = true) {
+    const overlay = {
+      composition: '',
+      setComposition: vi.fn(function (this: { composition: string }, text: string) {
+        this.composition = text;
+      }),
+      state: { visible },
+    };
+    Object.assign(app, { _localEchoEnabled: true, _localEchoOverlay: overlay });
+    return overlay;
+  }
+
+  it('routes the preview into the local echo overlay when local echo is on', () => {
+    const { app, helpers, previewNodes, createdControllers } = createPreviewHarness();
+    const overlay = withOverlay(app);
+    app._initMobileImePreview();
+    const callbacks = createdControllers[0].callbacks;
+    callbacks.render({ text: '天気', phase: 'provisional' });
+    expect(overlay.setComposition).toHaveBeenLastCalledWith('天気');
+    expect(previewNodes[0]).toMatchObject({ textContent: '', hidden: true });
+    expect(helpers.classList.contains('codeman-ime-preview-owned')).toBe(true);
+    callbacks.clear();
+    expect(overlay.setComposition).toHaveBeenLastCalledWith('');
+    expect(helpers.classList.contains('codeman-ime-preview-owned')).toBe(false);
+  });
+
+  it('uses the span when the overlay cannot place the composition (no prompt found)', () => {
+    const { app, previewNodes, createdControllers } = createPreviewHarness();
+    const overlay = withOverlay(app, false);
+    app._initMobileImePreview();
+    createdControllers[0].callbacks.render({ text: '天気', phase: 'provisional' });
+    expect(overlay.composition).toBe('');
+    expect(previewNodes[0]).toMatchObject({ textContent: '天気', hidden: false });
+  });
+
+  it('uses the span, not the overlay, when local echo is off or handed back to PTY echo', () => {
+    const off = createPreviewHarness();
+    const offOverlay = withOverlay(off.app);
+    Object.assign(off.app, { _localEchoEnabled: false });
+    off.app._initMobileImePreview();
+    off.createdControllers[0].callbacks.render({ text: 'かな', phase: 'provisional' });
+    expect(offOverlay.setComposition).not.toHaveBeenCalled();
+    expect(off.previewNodes[0]).toMatchObject({ textContent: 'かな', hidden: false });
+
+    const passthrough = createPreviewHarness();
+    const passOverlay = withOverlay(passthrough.app);
+    Object.assign(passthrough.app, { _echoPassthroughSessions: new Set(['session-a']) });
+    passthrough.app._initMobileImePreview();
+    passthrough.createdControllers[0].callbacks.render({ text: 'かな', phase: 'provisional' });
+    expect(passOverlay.setComposition).not.toHaveBeenCalled();
+    expect(passthrough.previewNodes[0]).toMatchObject({ textContent: 'かな', hidden: false });
+  });
+
   it('uses the terminal foreground and opaque background while mirroring native composition font metrics', () => {
     const { app, compositionView, previewNodes, createdControllers } = createPreviewHarness({
       themeForeground: '#1f2328',