ui: the node editor offers the de-noise node and persists its window

The form gains a `median samples` row, shown only for `DeNoise`; the corner row widens to
anything that reads `fc` (`math_op_uses_fc()`) while `q` stays band-pass-only, and the wiring
row keeps its place at the end so the form's order is unchanged for every existing op.

The session format gains a thirteenth field, `med_n`, after `y`. A twelve-field node -- a file
written before this node existed -- still loads, with `med_n` left at the struct's default,
which is what such a file meant; the reader's `return number(...) && ...` chain had to become a
guarded sequence so the thirteenth field could be read conditionally.

The math probe enables the channel it builds its graphs from, the same self-sufficiency rule
the noise and measurement probes already follow: `enable_all_probe` ends by switching one
signal off, and with `sine` gone the graphs landed on `square`, changing a documented line for
reasons unrelated to the math.

Readings: `[math-probe] 2 math channels on 'sine' and 'noise'`, `pane add: 'math3' sources
'sine'`, `graph edit: stored_slots 31000 -> 32000`, `dialog build: nodes=2, output after add=1`
and the rest of the suite unchanged; no `Gtk-CRITICAL`.

src/ui.cpp

@@ -2140,6 +2140,14 @@ void open_math_dialog(App& app, int edit) {
     gtk_box_append(GTK_BOX(form), clamp_rows);
   }
 
+  // Odd, 3..9. The engine rounds up to odd anyway, but a step of 2 means the widget never offers an even
+  // one.
+  d->med = gtk_spin_button_new_with_range(3.0, 9.0, 2.0);
+  gtk_spin_button_set_digits(GTK_SPIN_BUTTON(d->med), 0);
+  gtk_spin_button_set_value(GTK_SPIN_BUTTON(d->med), 5.0);
+  g_signal_connect(d->med, "value-changed", G_CALLBACK(on_math_param_changed), d);
+  gtk_box_append(GTK_BOX(form), math_row("median samples", d->med, &d->rows[6]));
+
   d->inputs = gtk_label_new("");
   gtk_label_set_wrap(GTK_LABEL(d->inputs), true);
   gtk_widget_add_css_class(d->inputs, "dim-label");
@@ -2159,7 +2167,7 @@ void open_math_dialog(App& app, int edit) {
   gtk_widget_set_visible(d->problem, FALSE);
   gtk_box_append(GTK_BOX(form), d->problem);
 
-  // A menu of the twelve ops rather than one button that always makes a gain: building anything
+  // A menu of the thirteen ops rather than one button that always makes a gain: building anything
   // else was add-then-change-the-dropdown, which is two steps for the common case.
   GtkWidget* add = gtk_menu_button_new();
   gtk_menu_button_set_label(GTK_MENU_BUTTON(add), "add node");