Fix virtual channels not appearing in Plot Builder

Root cause: PlotWindow deepcopied _chart._cfg on init, so refresh_channels()
operated on a stale copy. Also, when _win_plot was None, _on_derived_changed
did nothing to prepare the config for when it opened.

Fix:
- refresh_channels() now owns the derived-pane sync: adds panes for new
  virtual channels (updating BSP tree), removes panes for deleted ones
- _open_plot always passes current _chart._cfg reference then calls
  refresh_channels() so derived panes sync on every open
- _on_derived_changed syncs cfg reference before calling refresh_channels()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions