ui: the sidebar's tabs become one dock; Measure and Signals by keyboard section; session keeps the dock's state

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

src/layout_store.hpp

@@ -73,9 +73,10 @@ struct LayoutState {
   /// stream and mean nothing restored against another one.
   bool measure_stats = false;
 
-  /// Which sidebar tab was showing: "measure" or "signals". The one thing the panel persists --
-  /// no revealer states remain, and the 320 px width is the `.ui`'s.
-  std::string side_tab = "signals";
+  /// The dock's shown state (the user's last word on the toolbar's Dock button). The one thing the dock
+  /// persists -- its 320 px width is the `.ui`'s. An older file's `side_tab` key is not read: the parser
+  /// ignores keys it does not know.
+  bool dock_shown = true;
 
   /// The measurements pane, by signal *name* and with the type and options a row was built with.
   std::vector<Measure> measures;