fix: the trigger stays editable while it is off, and the locked pane says "stream"

Two things the lock got wrong, and they are the same mistake seen twice: trigger-off was treated as "the
pane is disabled".

**The configuration was unreachable exactly when it was wanted.** `on_trigger_toggled()` set the trigger
*body* insensitive with the mode — a rule carried over from the five-revealer sidebar, where a closed pane
had nothing to edit. A trigger is configured *before* it is switched on, so switching it off to change it,
or to read what it is set to, is the ordinary way to work. The body is now never disabled; only the
*cursors'* readout still is, which is the pane whose content has nothing to show with the instruments off.

**The locked pane described a trigger that was not running.** With trigger mode off the summary is one
line — `mode: stream (trigger off)` — because the four lines it used to show (signal, condition, level,
mode) describe a trigger that is not there. With the trigger on it is those four plus whatever is not at
its default (`noise band`, `HF reject`, `pre-trigger`, `hold-off`).

The reading changes with it: `trigger panel sensitive …=1` is now a **regression guard** rather than a
state — the trigger's body is always live, so a `0` there means the settings have been locked out again.

Verified in both states, from sessions whose only difference is `trigger_enabled`:

    panes: trigger=locked signals=locked body_visible=0 summary='mode: stream (trigger off)'
    panes: trigger=locked signals=locked body_visible=0 summary='signal: square | condition: window, rising | level: 0.3 .. 0.6 | mode: single | pre-trigger: 20 %'
    trigger panel sensitive before the probe toggled it: 1

The locked trigger pane is 72 px with the trigger off and 192 px with it on. Docs: the invariants' rule
for which body is disabled and why the trigger's is not, the `summary`'s two forms with what each means,
the README's deviation note, and the second `panes:` line renamed to say what it is — the line as the
probe found it, because only a run whose *session* has the trigger off shows the stream form.

src/ui.cpp

@@ -4852,11 +4852,10 @@ void on_activate(GtkApplication* application, gpointer user) {
   app->signal_panel = build_signal_panel(*app);
   gtk_box_append(GTK_BOX(signals_page), app->signal_panel);
 
-  // Both instruments start off, so both *bodies* start insensitive -- stated here by the one place
-  // that knows the default, rather than left to whichever widget happened to be built first. The
-  // bodies and not the panels: each panel's heading carries the switch that turns it on, and a
-  // control inside an insensitive parent cannot be clicked.
-  gtk_widget_set_sensitive(app->trigger_body, FALSE);
+  // The cursors start off, so their readout starts insensitive -- stated here by the one place that
+  // knows the default, and the *body* and not the panel, because the panel's heading carries the
+  // switch that turns it back on. The trigger's body is deliberately *not* disabled: a trigger is
+  // configured before it is switched on, so its settings have to stay editable while it is off.
   gtk_widget_set_sensitive(app->cursors_body, FALSE);
   gtk_widget_set_sensitive(app->cursors_reset, FALSE);
 
@@ -5790,6 +5789,8 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 app.dbg_trigger_locked ? "locked" : "config",
                 app.dbg_signals_locked ? "locked" : "config", app.dbg_trigger_body_visible,
                 app.dbg_trigger_summary.c_str());
+    std::printf("[udpscope]   panes: the same line as the probe found it: '%s'\n",
+                app.dbg_trigger_summary_early.c_str());
     std::printf("[udpscope]   panel switches: trigger=%d/%d cursors=%d/%d drove_toolbar=%d "
                 "(pane switch / header bar button; `drove_toolbar` is the pane's own switch having "
                 "moved the button)\n",