ui: Arm is disabled with trigger mode off, and the redundant Go-live button is gone

**`Arm` is now disabled while trigger mode is off**, and labelled `Arm` again: it is the one control in
that pane that is *runtime* rather than setting, so with no engine running there is nothing to arm.
It is also the only thing in the body that stays disabled — the settings themselves stay editable, which
is the point of the previous commit. `arm sensitive with the trigger off=0` is the reading.

**`Go live (trigger off)` is deleted.** Its handler was `trigger_toggle → FALSE`, i.e. exactly the
toolbar's trigger switch and the pane's own switch, which is what it was for: "the way out of trigger
mode, from inside the panel that froze the view: it is the toolbar's trigger switch, so there is exactly
one piece of state." With that switch now in the pane's own heading, the button was the same control
drawn a third time. The status line that told users about it says "or switch the trigger off to stream"
instead. The events pane's own `events_live` button is a different button on a different path — the file
view restore — and is untouched.

**And one silent regression this work uncovered.** The `trigger panel:` frame reading had been walking the
*wrapper* box the app puts around the `.ui`'s panel (heading row + locked summary + panel) ever since the
heading moved out of the file in the lock commit, so it reported `boxes=2 … search_inside=0/4` against a
panel that was entirely correct. It reads `trigger_body` — the file's own panel — again, and now says so in
its comment, because a reading that can silently start describing the wrong widget is the failure it exists
to catch.

Verified: `trigger panel: boxes=15 controls_inside=15/15 search_widgets=4/4 search_inside=4/4` (the
documented shape, restored); `arm sensitive with the trigger off=0`; `trigger panel sensitive …=1`;
`panes:`/`panel switches:`/`pane edits:` unchanged; `make test-ui` 14 files; zero `Gtk-CRITICAL`.

src/ui_app.hpp

@@ -239,6 +239,9 @@ struct App {
   /// parent cannot be clicked, so a switch that went insensitive with its own pane could never turn
   /// it back on.
   int dbg_panel_switch_live = -1;
+  /// The Arm button, read with the trigger off: it is the pane's one runtime control, so it is the one
+  /// thing in the body that *is* disabled with the mode.
+  int dbg_arm_sensitive = -1;
   int dbg_switcher_policy = -1;
   /// Whether `name_side_tabs()` has named the pills: it can only run once the switcher is mapped.
   bool side_tabs_named = false;
@@ -1347,7 +1350,6 @@ void on_layout_drag_begin(GtkGestureDrag* gesture, double x, double y, gpointer
 void on_layout_drag_update(GtkGestureDrag* gesture, double offset_x, double offset_y, gpointer user);
 void on_layout_drag_end(GtkGestureDrag* gesture, double offset_x, double offset_y, gpointer user);
 void on_trigger_toggled(GtkToggleButton* button, gpointer user);
-void on_go_live(GtkButton*, gpointer user);
 plot::u32 measure_color(std::size_t index);
 bool visible_samples(App& app, std::size_t channel, std::vector<double>& time, std::vector<double>& value);
 int measure_channel(App& app, const std::string& signal);