- Extract navigation state and logic into navigation-store.ts for
testability and separation of concerns
- NavigationContext now only wraps the store as a Solid provider
- Remove SIDEBAR_PANE from the model; depth-tabs have a single
focusable content pane (center/current column)
- Split LayerGraph and page imports into layer-graph.ts so the
navigation utils stay free of JSX (unit-testable)
- Update Shell keybind dispatch: remove sidebar pane handling,
simplify swipe to fixed-pane tabs only, clarify depth-tab h/l
- Add nav-model.test.ts covering depth stack, tab/pane focus, and
selection semantics
