fix(split-pane): gate app-level chords out of Pane B, address Ark0N's third pass

Pane B had no attachCustomKeyEventHandler of its own, so the document
capture-phase shortcut handler's preventDefault() (which does not stop
xterm) left Ctrl+K/Alt+1/Alt+B ALSO writing their raw byte/escape
sequence into Pane B's live PTY on top of whatever the app action did
to Pane A. Pane B now installs the same registry-aware gates the
primary pane's own attachCustomKeyEventHandler uses. Ctrl+V is left on
xterm's default paste — no image-paste trap to route it to.

Plus the rest of the review's smaller items:
- Narrowing the window past the desktop gate now closes an open split
  instead of leaving it stranded on screen.
- Split is refused while a web tab is active (activeWebviewId), which
  used to open Pane B's socket behind a hidden container.
- Pane B now handles the server's `{t:'r'}` refresh frame via a shared
  _loadBuffer() helper (also used by connect()), instead of ignoring it.
- The divider drag now uses pointer events + setPointerCapture (mirrors
  tab-rail-resize.js), a button!==0 guard, preventDefault, and a
  body.split-pane-resizing cursor/selection lock — a plain mousedown
  drag selected the text under the cursor as it crossed both terminals.
- Pane B's close control and the picker rows are real <button>s now
  (keyboard-reachable), with matching CSS chrome resets.
- Dropped the redundant CodemanBase.base prefix on the buffer fetch
  (the global fetch wrapper already applies it).
- data-preview-order for the Split settings chip moved from a collision
  with Ultracode Agents (both 15/12) to 11.5, matching its real
  position between Multi-monitor and Ultracode Agents in the header;
  widened test/app-settings-structure.test.ts's regex to allow the
  decimal (Number() already parses it fine for the preview sort).
- Added zh-CN i18n entries for the Split button and empty-picker text.
- Dropped the stray unused `vi` import Ark0N flagged as unrelated to
  this feature (vitest's `globals: true` makes it ambient anyway).
- Documented the fix and the deliberate no-cid/seq choice in the
  split-pane-sessions architecture-invariants entry.

Added a real-Chromium regression test asserting Ctrl+K/Alt+1/Alt+B
dispatched at Pane B's own textarea send no `{t:'i'}` frame over its
WebSocket. Full CI gate green (409 files, 7736 tests) plus all 8
split-pane browser tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -404,7 +404,7 @@ Anatomy: `.set-shell` → `.set-shell-head` (title + `.set-head-actions`) + `.se
 
 ### Split-pane sessions
 
-**Split-pane sessions** (`showSplitButton`, header button, default OFF, per-device like `showFileViewerButton` — not in `SettingsUpdateSchema`, `displayKeys` in settings-ui.js): shows two live sessions side-by-side in one Codeman window. Pane A is the untouched, existing singleton terminal (`this.terminal`/`this._ws` in terminal-ui.js); Pane B is a new, independent `SplitTerminalPane` (terminal-split.js) with its own xterm instance and its own `/ws/sessions/:id/terminal` WebSocket. ⚠️ **Pane B is deliberately plainer than Pane A** — no local-echo overlay, no CJK IME, no touch/mobile handlers, no keyboard accessory bar — since this is a desktop-only feature (a split view needs a wide viewport) and those features exist for mobile/touch input; `.btn-split` is hard-hidden on phones in mobile.css regardless of the setting, and the per-device setting means turning it on at a desk can never sync it onto a phone in the first place. No persistence: closing the browser tab or reloading always returns to the normal single-pane view; there is no localStorage key for split state. ⚠️ Splitting a session against itself is disallowed (the picker excludes the active session), **as is splitting against a popped-out (detached) session** — `buildSplitPickerSessions()` excludes `detachedSessions` because a detached session's own window is already claiming its PTY size, and `MAX_WS_PER_SESSION` needs no change since Pane A/B are always two different sessions. ⚠️ Either pane's session ending (deleted locally or from another client) auto-collapses the split — Pane A's session ending promotes Pane B to the new single pane via `selectSession(id, { auto: true })` (an app-driven selection, so it must not spend the session's idle alert — see the Approvals Inbox note above), never by trying to hot-swap the lightweight `SplitTerminalPane` object into the primary singleton state. ⚠️ Pane B refits on every window/sidebar/tab-rail resize via the SAME trailing-edge `ResizeObserver` callback that resizes Pane A (`throttledResize` in terminal-ui.js) — it only ever measured Pane A's own container, so without an explicit `this._splitPane?.fit()` call there Pane B silently kept its stale PTY size through every resize that did not happen to be a divider drag. ⚠️ A dropped WebSocket leaves Pane B visibly dead (a message written into its own xterm buffer) rather than silently swallowing keystrokes with nothing on screen to explain why — there is no reconnect logic for v1, matching the "deliberately plainer than Pane A" design. Related but distinct: `detachSession()` already opens one session in a separate OS-level browser window (`isSoloWindow`) — that is prior art for "two sessions visible at once" but not for one window with a draggable in-page divider, which is what this feature adds. Design: `docs/split-pane-sessions-plan.md`.
+**Split-pane sessions** (`showSplitButton`, header button, default OFF, per-device like `showFileViewerButton` — not in `SettingsUpdateSchema`, `displayKeys` in settings-ui.js): shows two live sessions side-by-side in one Codeman window. Pane A is the untouched, existing singleton terminal (`this.terminal`/`this._ws` in terminal-ui.js); Pane B is a new, independent `SplitTerminalPane` (terminal-split.js) with its own xterm instance and its own `/ws/sessions/:id/terminal` WebSocket. ⚠️ **Pane B is deliberately plainer than Pane A** — no local-echo overlay, no CJK IME, no touch/mobile handlers, no keyboard accessory bar — since this is a desktop-only feature (a split view needs a wide viewport) and those features exist for mobile/touch input; `.btn-split` is hard-hidden on phones in mobile.css regardless of the setting, and the per-device setting means turning it on at a desk can never sync it onto a phone in the first place. No persistence: closing the browser tab or reloading always returns to the normal single-pane view; there is no localStorage key for split state. ⚠️ Splitting a session against itself is disallowed (the picker excludes the active session), **as is splitting against a popped-out (detached) session** — `buildSplitPickerSessions()` excludes `detachedSessions` because a detached session's own window is already claiming its PTY size, and `MAX_WS_PER_SESSION` needs no change since Pane A/B are always two different sessions. ⚠️ Either pane's session ending (deleted locally or from another client) auto-collapses the split — Pane A's session ending promotes Pane B to the new single pane via `selectSession(id, { auto: true })` (an app-driven selection, so it must not spend the session's idle alert — see the Approvals Inbox note above), never by trying to hot-swap the lightweight `SplitTerminalPane` object into the primary singleton state. ⚠️ Pane B refits on every window/sidebar/tab-rail resize via the SAME trailing-edge `ResizeObserver` callback that resizes Pane A (`throttledResize` in terminal-ui.js) — it only ever measured Pane A's own container, so without an explicit `this._splitPane?.fit()` call there Pane B silently kept its stale PTY size through every resize that did not happen to be a divider drag. ⚠️ A dropped WebSocket leaves Pane B visibly dead (a message written into its own xterm buffer) rather than silently swallowing keystrokes with nothing on screen to explain why — there is no reconnect logic for v1, matching the "deliberately plainer than Pane A" design. Related but distinct: `detachSession()` already opens one session in a separate OS-level browser window (`isSoloWindow`) — that is prior art for "two sessions visible at once" but not for one window with a draggable in-page divider, which is what this feature adds. ⚠️ Pane B installs its own `attachCustomKeyEventHandler` gating the same three app-level chords Pane A's own handler gates (command palette, Alt+1-9/[/] tab nav, Alt+B sidebar toggle) — without it the document capture-phase handler's `preventDefault()` (which never stops xterm) let each chord ALSO write its raw byte/escape sequence into Pane B's live PTY on top of whatever the app action did to Pane A (COD-153). Ctrl+V stays on xterm's own default paste, since Pane B has no image-paste trap to route it to. Pane B's input frames deliberately carry no `cid`/`seq` (`ws-routes.ts` supports that), matching the no-overlay/no-IME "deliberately plainer" list above, since it has no exactly-once delivery layer to key them against. Design: `docs/split-pane-sessions-plan.md`.
 
 ### Gesture control: the setting