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_app.hpp

@@ -978,9 +978,10 @@ struct MathDialog {
   GtkWidget* offset = nullptr;
   GtkWidget* fc = nullptr;
   GtkWidget* q = nullptr;
+  GtkWidget* med = nullptr;  ///< MathNode::med_n
   GtkWidget* lo = nullptr;
   GtkWidget* hi = nullptr;
-  GtkWidget* rows[6] = {nullptr, nullptr, nullptr, nullptr, nullptr, nullptr};
+  GtkWidget* rows[7] = {nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr};
   GtkWidget* inputs = nullptr;
   /// The validation line and the button it governs. Both are set from `math_dialog_sync()` and from
   /// nowhere else, so what the line says and what the button allows cannot disagree.