ui: the Dock toggle carries the ruler icon, the dock being the readouts

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

docs/claude/invariants-gtk-and-chrome.md

@@ -59,7 +59,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   are the five action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
   user; `udpscope-measure-symbolic` **stayed**, because the Preferences window's Plot page carries it --
   an icon with a live user is not an orphan, whatever the toggle that introduced it did -- and it is
-  the Preferences window's icon.
+  the Preferences window's icon and the **Dock toggle's** (the ruler: the dock is the readouts).
 - **A shortcut table row's `accel` is GTK accelerator syntax, and a pointer gesture has
   none.** `src/shortcuts.cpp`'s strings are handed to `AdwShortcutsItem`, which parses them
   with `gtk_accelerator_parse()` — so they are `"space"`, `"<Control>e"`, `"<Control>slash"`,

src/ui/header-bar.ui

@@ -46,8 +46,8 @@
     </child>
     <child type="end">
       <object class="GtkToggleButton" id="sidebar_toggle">
-        <property name="icon-name">sidebar-show-symbolic</property>
+        <property name="icon-name">udpscope-measure-symbolic</property>
-        <property name="tooltip-text" translatable="yes">show the dock</property>
+        <property name="tooltip-text" translatable="yes">show the readouts dock (cursors, measurements, events)</property>
         <property name="active">1</property>
       </object>
     </child>