fix: the [edit] buttons' icons, and the locked pane says four lines not one

Two things the lock shipped wrong.

**The "done" icon did not exist.** `emblem-ok-symbolic` is a breeze icon; Adwaita — which is what this
app runs under — does not have it, so the button that leaves configuration mode drew a missing-image
placeholder, i.e. nothing. Both names are now Adwaita's own: `document-edit-symbolic` (the pencil: edit)
and `object-select-symbolic` (the check: done). This is exactly the silent failure `toolbar icons:`
exists for, so the two `[edit]` buttons get the same treatment: `pane edits: locked=…(ok) config=…(ok)`
is read by clicking into configuration mode and back, so one line carries both names and
`gtk_icon_theme_has_icon()`'s verdict for each. The invariants now carry the rule, including the trap
that a name found under `breeze` is not an answer.

**The locked pane was too compact.** One line could not hold what a user opens the pane for, so it is now
a small definition list: `signal`, `condition`, `level`, `mode` on four lines, and after that only the
figures that are *not* at their defaults — `noise band`, `HF reject`, `pre-trigger`, `hold-off` — because
a line reading "band 0" is a line that says nothing. The channel is resolved from the acquired channels
and the condition quotes the figures its own shape reads (a width and its comparison, two levels, or a
timeout), never one it ignores. The pane is ~192 px tall locked against 580 in configuration mode.

One self-inflicted critical on the way: the probe read the summary with `gtk_label_get_text()`, and the
container is a grid now — `Gtk-CRITICAL: assertion 'GTK_IS_LABEL (self)' failed`. The text comes from the
sync that writes it instead.

Verified: `pane edits: locked=document-edit-symbolic(ok) config=object-select-symbolic(ok)`;
`panes: trigger=locked signals=locked body_visible=0 summary='signal: square | condition: window, falling
| level: 0.3 .. 0.6 | mode: single | noise band: 0.2 | pre-trigger: 20 % | hold-off: 1000 ms'`;
`make test-ui` 14 files; zero `Gtk-CRITICAL` and no missing icon.

src/ui_probe.cpp

@@ -2322,13 +2322,24 @@ gboolean exit_probe_step1(gpointer user) {
   App* a = static_cast<App*>(user);
   // Record the panel states before the toggles are exercised.
   a->dbg_trigger_sensitive_default = gtk_widget_get_sensitive(a->trigger_body) ? 1 : 0;
+  // The two [edit] buttons, in each mode: the icon is the only thing that says which mode the pane is
+  // in, and an icon the theme does not have draws nothing at all.
+  const auto pane_icon = [](GtkWidget* b) {
+    const char* n = b != nullptr && GTK_IS_BUTTON(b) ? gtk_button_get_icon_name(GTK_BUTTON(b)) : nullptr;
+    return std::string(n != nullptr ? n : "-");
+  };
+  a->dbg_pane_edit_icon = pane_icon(a->trigger_edit);
+  if (a->trigger_edit != nullptr) {
+    g_signal_emit_by_name(a->trigger_edit, "clicked");  // into configuration mode
+    a->dbg_pane_view_icon = pane_icon(a->trigger_edit);
+    g_signal_emit_by_name(a->trigger_edit, "clicked");  // and back to the view
+  }
   a->dbg_trigger_locked = a->trigger_locked ? 1 : 0;
   a->dbg_signals_locked = a->signals_locked ? 1 : 0;
   a->dbg_trigger_body_visible =
       a->trigger_body != nullptr && gtk_widget_get_visible(a->trigger_body) ? 1 : 0;
-  if (a->trigger_summary != nullptr) {
-    a->dbg_trigger_summary = gtk_label_get_text(GTK_LABEL(a->trigger_summary));
-  }
+  // 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_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)