refactor(terminal): seams for a second terminal pane (input socket map, targeted links, copy, paste)

No behavior change. Prepares the split pane's second terminal (and later
grid tiles) to share what today only the primary terminal has:

- _inputSocketFor/_registerInputSocket/_unregisterInputSocket: the
  exactly-once input queue, its ACK handling and the redelivery sweep now
  deliver over any registered socket bound to a session, not only
  this._ws. ACKs are routed by the receiving socket's session; silence is
  judged per socket; a stale handle cannot unregister its replacement.
- registerFilePathLinkProvider, cleanedTerminalSelection,
  copyTerminalSelection and _handleImagePaste take an optional target
  terminal and session (defaults: the primary pane).
- _focusedPane() is the one place to ask which pane the keyboard is in
  (primary only, for now); _forEachTile() replaces the _splitPane special
  cases in the font, family, weight, skin and resize paths.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -867,6 +867,11 @@ class CodemanApp {
     // writes when the freshly computed descriptor is identical (COD-136).
     this._lastIndicatorDescriptor = null;
     this._postDraining = new Set(); // sessionIds with an in-flight POST drainer
+    // Terminal sockets OTHER than the primary one (`this._ws`), keyed by the
+    // session they are bound to: the split pane's second terminal registers its
+    // socket here so its input rides the same exactly-once queue. Values are
+    // `{ ws, lastRecvAt }` handles owned by that terminal (see _inputSocketFor).
+    this._extraInputSockets = new Map();
     this._persistReliableTimer = null;
     this._reliableAckTimeoutMs = 4000; // unacked WS frame older than this ⇒ socket likely dead
     this._reliableMaxBytes = 256 * 1024; // cap on the persisted backlog
@@ -3397,7 +3402,7 @@ class CodemanApp {
         } else if (msg.t === 'ia') {
           // Input ACK — the server applied (or deduped) this seq; drop it from
           // the durable queue so it can never be re-delivered/lost.
-          this._onWsInputAck(msg.seq, msg);
+          this._onWsInputAck(msg.seq, msg, sessionId);
         } else if (msg.t === 'zc') {
           // Resize confirm — the geometry the PTY actually holds, which is not
           // always the one this client asked for (issue #464).
@@ -3579,6 +3584,49 @@ class CodemanApp {
     this._reliableSend(sessionId, input, useMux);
   }
 
+  /**
+   * The OPEN terminal socket bound to `sessionId`, or null: the primary socket
+   * first, then one registered by a second terminal (the split pane). The
+   * delivery layer below asks this instead of reading `this._ws` directly, so a
+   * second terminal's input gets the same exactly-once queue, ACKs and
+   * half-open detection as the primary's.
+   *
+   * Returns `{ ws, lastRecvAt }`: `lastRecvAt` is the socket's last received
+   * frame, which `_redeliverSweep` reads to tell a dead socket from a slow ACK.
+   */
+  _inputSocketFor(sessionId) {
+    if (!sessionId) return null;
+    if (this._ws && this._ws.readyState === WebSocket.OPEN && this._wsSessionId === sessionId) {
+      return { ws: this._ws, lastRecvAt: this._wsLastRecvAt };
+    }
+    const handle = this._extraInputSockets?.get(sessionId);
+    if (handle && handle.ws && handle.ws.readyState === WebSocket.OPEN) return handle;
+    return null;
+  }
+
+  /**
+   * Register a second terminal's socket for `sessionId` (call from its onopen).
+   * `handle` is `{ ws, lastRecvAt }`, owned by the caller, which must bump
+   * `handle.lastRecvAt` on every received frame. Stamped here so a socket that
+   * just opened does not look silent to the redelivery sweep, which would
+   * otherwise force-close it as half-open on the first stale record.
+   */
+  _registerInputSocket(sessionId, handle) {
+    if (!sessionId || !handle) return;
+    handle.lastRecvAt = Date.now();
+    if (!this._extraInputSockets) this._extraInputSockets = new Map();
+    this._extraInputSockets.set(sessionId, handle);
+  }
+
+  /**
+   * Drop a registration, but only the one `handle` made: a replacement socket
+   * registers its own handle before the old one's close lands, and that late
+   * close must not unregister the socket that replaced it.
+   */
+  _unregisterInputSocket(sessionId, handle) {
+    if (this._extraInputSockets?.get(sessionId) === handle) this._extraInputSockets.delete(sessionId);
+  }
+
   /**
    * Fire-and-forget input for EPHEMERAL, loss-tolerant streams (e.g. wheel-scroll
    * reports). Unlike _sendInputAsync, this never enters the durable seq/ACK queue,
@@ -3590,9 +3638,10 @@ class CodemanApp {
    */
   _sendInputEphemeral(sessionId, input) {
     if (!sessionId || !input) return;
-    if (this._ws && this._ws.readyState === WebSocket.OPEN && this._wsSessionId === sessionId) {
+    const sock = this._inputSocketFor(sessionId);
+    if (sock) {
       try {
-        this._ws.send(JSON.stringify({ t: 'i', d: input }));
+        sock.ws.send(JSON.stringify({ t: 'i', d: input }));
         return;
       } catch {
         // socket died mid-send — fall through to a best-effort POST
@@ -3646,11 +3695,12 @@ class CodemanApp {
     // over the single ordered stream. They stay pending until the server ACKs
     // them ({t:'ia'}); a frame swallowed by a half-open socket is re-sent after
     // the sweep force-reconnects (which resets sentAt=0 in _onWsReady).
-    if (this._ws && this._ws.readyState === WebSocket.OPEN && this._wsSessionId === sessionId) {
+    const sock = this._inputSocketFor(sessionId);
+    if (sock) {
       for (const rec of list) {
         if (rec.sentAt !== 0) continue;
         try {
-          this._ws.send(JSON.stringify({ t: 'i', d: rec.data, seq: rec.seq, cid: this._clientId }));
+          sock.ws.send(JSON.stringify({ t: 'i', d: rec.data, seq: rec.seq, cid: this._clientId }));
           rec.sentAt = Date.now();
           rec.tries++;
         } catch {
@@ -3671,7 +3721,7 @@ class CodemanApp {
           if (!cur || cur.length === 0) break;
           // If the WebSocket came back mid-drain, yield to it (the acked stream)
           // so we don't redundantly re-POST what onopen is already re-sending.
-          if (this._ws && this._ws.readyState === WebSocket.OPEN && this._wsSessionId === sessionId) {
+          if (this._inputSocketFor(sessionId)) {
             break;
           }
           const rec = cur[0];
@@ -3759,8 +3809,10 @@ class CodemanApp {
    * called a duplicate is the mechanism working as designed — the original did
    * land — and re-sending it would type the same thing twice.
    */
-  _onWsInputAck(seq, msg) {
-    const sessionId = this._wsSessionId;
+  _onWsInputAck(seq, msg, sessionId = this._wsSessionId) {
+    // `sessionId` is the session of the socket the ACK arrived on: `{t:'ia'}`
+    // frames carry none, and with a second terminal's socket in play
+    // `this._wsSessionId` is no longer the only candidate.
     if (!sessionId || !Number.isInteger(seq)) return;
     if (msg && msg.err) {
       // Refused for good (e.g. over the size limit): retrying cannot help.
@@ -3805,19 +3857,20 @@ class CodemanApp {
     for (const sessionId of [...this._pendingDeliveries.keys()]) {
       const list = this._pendingDeliveries.get(sessionId);
       if (!list || list.length === 0) continue;
-      const isActiveWs =
-        this._ws && this._ws.readyState === WebSocket.OPEN && this._wsSessionId === sessionId;
-      if (isActiveWs) {
+      const sock = this._inputSocketFor(sessionId);
+      if (sock) {
         const oldest = list[0];
         // Only tear the socket down when the oldest unacked frame is stale AND the
         // socket has been silent for the timeout: a connection still delivering
         // output/ACKs is alive (the ACK is just behind), so force-closing it would
         // cause needless WS↔HTTP flapping. A truly half-open socket goes quiet.
+        // Silence is measured on THIS socket: the primary's last frame says
+        // nothing about a second terminal's connection, and the reverse.
         const stale = oldest && oldest.sentAt && Date.now() - oldest.sentAt > this._reliableAckTimeoutMs;
-        const silent = Date.now() - this._wsLastRecvAt > this._reliableAckTimeoutMs;
+        const silent = Date.now() - (sock.lastRecvAt || 0) > this._reliableAckTimeoutMs;
         if (stale && silent) {
           try {
-            this._ws.close(); // half-open: never recovers on its own — force reconnect
+            sock.ws.close(); // half-open: never recovers on its own — force reconnect
           } catch {
             /* ignore */
           }