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/header-bar.ui

@@ -3,7 +3,7 @@
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
   The header bar: the time-window control's slot, the trigger, pause and events group with the separator that
-  marks where the *view's* controls end, and the five at the far end.
+  marks where the *view's* controls end, and the three at the far end.
 
   The bar carries only the controls you reach for *while watching a signal*; everything you set up once lives
   in the overflow menu, which is code — its rows act on the `RowSet` directly and two of them are toggles
@@ -11,7 +11,7 @@
 
   The order is the file's, and it is the one thing here that is not self-evident: `pack_end()` lays its
   children out right to left, so a `<child type="end">` is added leftwards-from-the-right in document order
-  and the screen reads the reverse of it — cursors, measurements, math, sidebar, menu.
+  and the screen reads the reverse of it — cursors, sidebar, menu.
 
   The first start child is a slot and not the control: the time-window control is code (its menu's rows come
   from the buffer's limits), and `gtk_header_bar_pack_start()` appends *after* whatever a file has already
@@ -64,18 +64,6 @@
         <property name="active">1</property>
       </object>
     </child>
-    <child type="end">
-      <object class="GtkToggleButton" id="math_toggle">
-        <property name="icon-name">udpscope-math-symbolic</property>
-        <property name="tooltip-text" translatable="yes">channels derived from the others by a graph of operations: gain, absolute value, filters, sums and the rest. An enabled one is acquired, plotted and measured exactly like a signal</property>
-      </object>
-    </child>
-    <child type="end">
-      <object class="GtkToggleButton" id="measures_toggle">
-        <property name="icon-name">udpscope-measure-symbolic</property>
-        <property name="tooltip-text" translatable="yes">compute a value over the visible window of a signal, and optionally mark it on the plot</property>
-      </object>
-    </child>
     <child type="end">
       <object class="GtkToggleButton" id="cursors_toggle">
         <property name="icon-name">udpscope-cursors-symbolic</property>