fix(mobile): CJK input loss — IME state machine, focus routing, and Android InputConnection recovery

Three independent root causes of intermittent Chinese character loss
(English was unaffected because it bypasses the composition path):

1. input-cjk.js state machine: stuck _composing when compositionend never
   fires (WeChat/Sogou IMEs) silently swallowed all input; the deferred
   compositionend flush could reset the textarea mid-next-composition
   (cancels the live IME composition on iOS); the 100ms keydown-echo
   window discarded ANY input regardless of content.

2. Focus stealing: session-select / SSE-reconnect paths call
   terminal.focus() (15+ call sites), landing focus on xterm's hidden
   textarea; with the CJK onData gate active, everything typed there was
   swallowed. Fix: focus router in initTerminal routes ALL
   terminal.focus() calls to the CJK field while it is visible, plus a
   self-healing onData gate that reclaims focus when it swallows input.

3. Android InputConnection wedge (9-key IMEs + Chromium): the keyboard
   composes in its own UI but delivers zero DOM events. Fix: skip
   redundant textarea value/selection writes (they race IME session
   setup), and re-tapping the focused empty field forces a blur→focus
   cycle that restarts the input session.

Diagnostics: input-cjk.js now traces every IME event/flush decision into
the crash-diag breadcrumbs; /api/crash-diag stores beacons per page-load
id (iOS PWA reloads no longer wipe the trail, concurrent clients no
longer clobber each other) and flushes on visibilitychange.

Tests: test/input-cjk.test.ts (vm-sandbox, 9 cases incl. regression
guards for all three root causes).

src/web/public/terminal-ui.js

@@ -305,6 +305,25 @@ Object.assign(CodemanApp.prototype, {
       });
     }
 
+    // ── Focus router ──
+    // While the CJK field is visible, EVERY terminal.focus() call must land on
+    // the CJK field instead. Focusing xterm's hidden textarea in CJK mode sends
+    // the IME's output into a black hole: the keyboard composes normally, but
+    // onData is gated by cjkActive, so nothing reaches the field OR the PTY.
+    // Session select / SSE-reconnect restore paths call terminal.focus() and
+    // were silently stealing focus after every app switch on mobile (the
+    // intermittent "Chinese input goes nowhere" bug). One chokepoint here
+    // covers all ~15 call sites plus any future ones.
+    const _xtermFocus = this.terminal.focus.bind(this.terminal);
+    this.terminal.focus = () => {
+      const cjkEl = document.getElementById('cjkInput');
+      if (cjkEl?.classList.contains('cjk-input-visible')) {
+        cjkEl.focus();
+      } else {
+        _xtermFocus();
+      }
+    };
+
     // On mobile Safari, delay initial fit() to allow layout to settle
     // This prevents 0-column terminals caused by fit() running before container is sized
     const isMobileSafari =
@@ -627,7 +646,19 @@ Object.assign(CodemanApp.prototype, {
       // is on, because cjkActive stays true the whole time the field is visible.
       const isMouseReport = /^\x1b\[<\d+;\d+;\d+[Mm]$/.test(data);
       // CJK input has focus — block xterm from sending keystrokes to PTY
-      if (!isMouseReport && (window.cjkActive || document.activeElement?.id === 'cjkInput')) return;
+      if (!isMouseReport && (window.cjkActive || document.activeElement?.id === 'cjkInput')) {
+        // Self-heal: if the CJK field is visible but focus drifted to xterm's
+        // hidden textarea (e.g. something called terminal.focus()), everything
+        // typed lands HERE and is swallowed — keyboard shows the IME composing
+        // while both the CJK field and the terminal stay empty. Route focus
+        // back so the very next keystroke lands in the CJK field again.
+        const cjkEl = document.getElementById('cjkInput');
+        if (cjkEl?.classList.contains('cjk-input-visible') && document.activeElement !== cjkEl) {
+          _crashDiag.log('CJK regain-focus (onData swallowed input)');
+          cjkEl.focus();
+        }
+        return;
+      }
       if (this.activeSessionId) {
         // Filter terminal query replies generated by xterm.js itself.
         // Forwarding them through the WebSocket injects DA/DSR/CPR replies
@@ -1622,7 +1653,11 @@ Object.assign(CodemanApp.prototype, {
   // CJK textarea already provides visual feedback — bypass local echo
   // buffering so each composed word reaches the PTY immediately.
   _handleCjkInput(text) {
-    if (!this.activeSessionId) return;
+    if (!this.activeSessionId) {
+      _crashDiag.log(`CJK send DROP no-session len=${text.length}`);
+      return;
+    }
+    _crashDiag.log(`CJK send→${this.activeSessionId.slice(0, 8)} len=${text.length}`);
     this._sendInputAsync(this.activeSessionId, text);
   },