Editor: share the SwapManager along with shared buffers

A secondary GUI window shared the primary's buffer vector but attached
buffers through its own SwapManager. Closing the window destroyed that
manager while buffers still held its recorders, so the next edit called
into freed memory; Rehome through the wrong manager also dropped
journaling for the primary's buffers.

Audit finding M3.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

ImGuiFrontend.cc

@@ -378,7 +378,7 @@ GUIFrontend::OpenNewWindow_(Editor &primary)
 	ws->height    = h;
 
 	// Secondary editor shares the primary's buffer list
-	ws->editor.SetSharedBuffers(&primary.Buffers());
+	ws->editor.SetSharedBuffers(&primary.Buffers(), primary.Swap());
 	ws->editor.SetDimensions(primary.Rows(), primary.Cols());
 
 	// Open a new untitled buffer and switch to it in the new window.