ui: the sidebar is two tabs, and a toggle disables its pane

Replace the sidebar's five-panel scrolling column with an AdwViewStack
carrying two pages — Measure (cursors + measurements) and Signals
(trigger + math + channel list) — with an AdwViewSwitcher above it. The
trigger and cursors toggles change from "reveal/hide my pane" to
"enable/disable my feature, leaving the pane insensitive", following the
disable-not-hide principle. The keys probe loses a duplicate registration
that was toggling the sidebar back on before the exit probe captured its
state; the exit-probe captures now read adw_overlay_split_view_get_show_
sidebar() instead of gtk_widget_get_visible(), which AdwOverlaySplitView
never changes.

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

src/ui_math.cpp

@@ -18,11 +18,11 @@ void apply_math_channels(App& app) {
   // `set_math_channels()` takes the model's lock itself -- holding it here is a self-deadlock on a
   // non-recursive mutex, and it is exactly how the first version of the probe hung the app.
   app.model->set_math_channels(app.math);
-  // The structural rebuild above replaced the buffer and dropped its history. A stream's producer
-  // refills that; a file's nothing does, so the view is re-read -- and with the math set now in place
-  // the reload computes the derived channels over the file as it goes.
   reload_file_view(app);
   rebuild_math_pane(app);
+  // The math channels are rows of the signal list too -- `append_math_signals_locked()` puts them
+  // in `signals()` -- so the list is stale until it is rebuilt.
+  rebuild_signal_panels(app);
   schedule_layout_save(app);
 }