Both backends share one SelectionState owned by MainWindow and live
side by side in a QStackedWidget — switching is setCurrentWidget, so
no widget is destroyed mid-session (tearing a VTK window down leaves
dangling make-current callbacks). The choice persists in QSettings.
CanvasCapabilities declares per-backend gaps (force diagrams and
video export stay PyVista-only, both are documented and greyed out)
and the UI gates on capabilities rather than the backend name. The
architecture gate now allows the canvas_plotly package.
Auto-fixes (UP037, UP038, F401, I001, SIM105, RUF100) plus hand-fixes:
drop the now-unused `Union` imports, annotate mutable class attributes
with ClassVar, raise ValidationError instead of blind Exception in the
zero-length test, delete two dead locals, and bind the per-iteration
grid-builder closures to their loop variables (B023).
Catch-all for the intermixed residue of the unpushed otko-development
work ported into this tree: combinations/console-dock/quick-guide wiring
across commands, core, services, views and tests; repo-wide ruff-format
normalization; README/CONTRIBUTING updates; and the toolbar default
(both toolbars now open in the top area, quick guide text updated).
Splitting this further would require hunk-level surgery with low
confidence; the preceding commits in this branch isolate the
self-contained features.
