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_probe.cpp

@@ -2341,6 +2341,7 @@ gboolean exit_probe_step1(gpointer user) {
   // The summary's text comes from the sync that wrote it: the container is a grid of key/value rows,
   // and asking it for a label's text was itself a critical.
   a->dbg_trigger_summary_early = a->dbg_trigger_summary;  // as found, before this probe touches it
+  a->dbg_arm_sensitive = a->arm_button != nullptr && gtk_widget_get_sensitive(a->arm_button) ? 1 : 0;
   a->dbg_panel_switch_live =
       a->trigger_switch != nullptr && gtk_widget_get_sensitive(a->trigger_switch) &&
               a->cursors_switch != nullptr && gtk_widget_get_sensitive(a->cursors_switch)