ui: a slide-over panel over the plot's right edge, with its probe

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

src/ui/main-window.ui

@@ -3,7 +3,8 @@
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
   The main window's skeleton: the window, the split view that puts the sidebar beside the plot, the
-  events pane's divider, the plot column with the status line under it, and the sidebar's tab stack.
+  events pane's divider, the plot column (`plot_overlay` > `plot_box`, the slide-over panel being an overlay child added in code)
+  with the status line under it, and the sidebar's tab stack.
 
   The sidebar is an `AdwOverlaySplitView`, which is what a utility sidebar is in GNOME (as against
   `AdwNavigationSplitView`, which is for navigation lists). Three things follow from that, and they are
@@ -124,16 +125,27 @@
                 <property name="orientation">vertical</property>
                 <property name="hexpand">1</property>
                 <child>
-                  <object class="GtkBox" id="plot_box">
-                    <property name="orientation">vertical</property>
-                    <property name="homogeneous">1</property>
-                    <property name="spacing">4</property>
+                  <!-- The plot, and the slide-over panel that opens over its right edge. The panel is
+                       one opaque overlay child placed by `halign=end` and a 360 px width, never a
+                       full-area transparent child, which would be picked wherever its allocation is
+                       and swallow the plot's pan, drop and hover (docs/claude/invariants-plot-and-layout.md).
+                       The overlay is the only parent change: `plot_box` keeps its id and its content. -->
+                  <object class="GtkOverlay" id="plot_overlay">
                     <property name="hexpand">1</property>
                     <property name="vexpand">1</property>
                     <child>
-                      <object class="GtkPaned" id="plot_split">
-                        <property name="orientation">horizontal</property>
-                        <property name="shrink-end-child">1</property>
+                      <object class="GtkBox" id="plot_box">
+                        <property name="orientation">vertical</property>
+                        <property name="homogeneous">1</property>
+                        <property name="spacing">4</property>
+                        <property name="hexpand">1</property>
+                        <property name="vexpand">1</property>
+                        <child>
+                          <object class="GtkPaned" id="plot_split">
+                            <property name="orientation">horizontal</property>
+                            <property name="shrink-end-child">1</property>
+                          </object>
+                        </child>
                       </object>
                     </child>
                   </object>