fix(split-pane): keep Pane B's disconnected marker last when the socket closes mid-pull (#506 review)

- terminal-split.js: move the socket's close into _onSocketClosed(), which
  defers the marker while a history pull holds live output (_liveQueue);
  _pullHistory() records closedBefore and its finally writes the marker
  after the queue flush when the socket closed during the pull, replayed
  or not, so it never lands above held frames or between replay chunks
- tests: drive the real close path for a close mid-fetch ending in a skip,
  a downgrade or a failed fetch, a close during the chunked replay, and a
  close with no pull running; pin the onclose wiring in the static guard;
  describe the mid-fetch case on its own
- CLAUDE.md: turn the plain-text split-pane pointer into a link
- architecture-invariants.md: describe the deferred marker

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

src/web/public/terminal-split.js

@@ -293,19 +293,26 @@
       // normal while it quietly ate everything typed into it. v1 scope is
       // "say so", not reconnect — collapsing the split would lose the
       // user's place in Pane B's scrollback for a transient blip.
-      this.ws.onclose = () => {
-        this._wsReady = false;
-        this._wsClosed = true;
-        this._writeDisconnectedMarker();
-      };
+      this.ws.onclose = () => this._onSocketClosed();
 
       this.ws.onerror = () => {
         // onclose fires after onerror — cleanup happens there.
       };
     }
 
-    // Extracted so both onclose and a history-pull replay that lands on an
-    // already-closed socket can write it (see _pullHistory()'s finally block).
+    // The socket's close, split out of connect() so the tests can drive it.
+    // While a history pull is running the marker waits for the pull's finally
+    // block: written now, it would sit above the output the pull is still
+    // holding (flushed after it on a skip, a downgrade or a failed fetch) or
+    // land in the middle of a chunked replay.
+    _onSocketClosed() {
+      this._wsReady = false;
+      this._wsClosed = true;
+      if (!this._liveQueue) this._writeDisconnectedMarker();
+    }
+
+    // Extracted so both _onSocketClosed() and a history pull that ends on a
+    // closed socket can write it (see _pullHistory()'s finally block).
     _writeDisconnectedMarker() {
       this.terminal?.write('\r\n\x1b[2m[Pane B disconnected — close and reopen the split to reconnect]\x1b[0m\r\n');
     }
@@ -423,6 +430,8 @@
     // main thread) and replays it under the reader's current place. Holds the
     // single-flight flag across the fetch AND the replay, like _loadBuffer().
     async _pullHistory() {
+      // A close before the pull already wrote its marker; one during it did not.
+      const closedBefore = this._wsClosed;
       this._bufferLoading = true;
       this._liveQueue = [];
       let replayed = false;
@@ -491,14 +500,15 @@
           if (entry.clear) this.terminal?.clear();
           else this.terminal?.write(entry.data);
         }
-        // A replay's own `\x1bc` wipes the disconnected marker onclose wrote,
+        // A replay's own `\x1bc` wipes a marker written before the pull,
         // painting a fresh, current-looking history while onData keeps
-        // silently dropping every keystroke on the dead socket. Re-stamp it
-        // if the socket closed in either order (before the pull started, or
-        // while the fetch was in flight) — checked after the queue flush so
-        // it is the last thing on screen, matching what onclose would have
+        // silently dropping every keystroke on the dead socket, so re-stamp it
+        // after a replay. A close DURING the pull wrote no marker at all
+        // (_onSocketClosed() defers it while the queue is live), so write it
+        // whether or not this pull replayed. Checked after the queue flush so
+        // it is the last thing on screen, matching what the close would have
         // left had the pull never run.
-        if (replayed && this._wsClosed) this._writeDisconnectedMarker();
+        if (this._wsClosed && (replayed || !closedBefore)) this._writeDisconnectedMarker();
         this._endBufferLoad();
       }
     }