fix(editor): collapse the workspace split when the last buffer closes (T-459)

EditorRegistry.close() guarded its active-changed emit on `_activeId !=
null`, so closing the LAST buffer (active clears to null) emitted only
editor.closed — never the active-changed(id:null) the editor extension
listens for to call closeEditor(). editorOpen stayed true and the top
split sat orphaned over the Claude pane. Always emit active-changed when
the active buffer is removed, including the cleared-to-null case; the
slot renderer already collapses correctly once editorOpen flips false.

The existing extension test fabricated the null active-changed event, so
it passed despite the registry never emitting it — that gap is why the
bug shipped. Add a registry test that drives the real close() path, plus
a slot_host widget test asserting the split (drag handle) drops out and
the primary pane fills the column.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/editor/registry.dart

@@ -180,8 +180,13 @@ class EditorRegistry {
     if (buf == null) return;
     _pathToId.remove(buf.path);
     if (_activeId == id) {
+      // Promote the next buffer, or clear to null when this was the last one.
+      // Emit active-changed in BOTH cases: a null id is the signal the editor
+      // split collapses on (T-459). Guarding the emit on `_activeId != null`
+      // suppressed exactly the last-buffer-closed event, leaving editorOpen
+      // stuck true and the top split orphaned over the primary pane.
       _activeId = _buffers.values.isEmpty ? null : _buffers.values.first.id;
-      if (_activeId != null) _emitActive();
+      _emitActive();
     }
     _emit('editor.closed', {'id': id, 'path': buf.path});
   }