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/layout_store.cpp

@@ -366,7 +366,7 @@ std::string math_node_text(const MathNode& node) {
   std::ostringstream o;
   o << kind << '|' << node.source << '|' << node.in_a << '|' << node.in_b << '|' << node.gain << '|'
     << node.offset << '|' << node.fc << '|' << node.q << '|' << node.lo << '|' << node.hi << '|'
-    << node.x << '|' << node.y;
+    << node.x << '|' << node.y << '|' << node.med_n;
   return o.str();
 }
 
@@ -382,7 +382,9 @@ bool math_node_from_text(const std::string& text, MathNode& out) {
     }
   }
   f.push_back(cur);
-  if (f.size() != 12u) return false;
+  // Twelve fields is a session file written before the de-noise node existed; its `med_n` stays at the
+  // struct's default, which is what that file meant.
+  if (f.size() != 12u && f.size() != 13u) return false;
   const auto number = [&](std::size_t i, double& v) {
     try {
       v = std::stod(f[i]);
@@ -402,8 +404,16 @@ bool math_node_from_text(const std::string& text, MathNode& out) {
   out.source = f[1];
   out.in_a = static_cast<int>(in_a);
   out.in_b = static_cast<int>(in_b);
-  return number(4, out.gain) && number(5, out.offset) && number(6, out.fc) && number(7, out.q) &&
-         number(8, out.lo) && number(9, out.hi) && number(10, out.x) && number(11, out.y);
+  if (!(number(4, out.gain) && number(5, out.offset) && number(6, out.fc) && number(7, out.q) &&
+        number(8, out.lo) && number(9, out.hi) && number(10, out.x) && number(11, out.y))) {
+    return false;
+  }
+  if (f.size() == 13u) {
+    double med = 0.0;
+    if (!number(12, med)) return false;
+    out.med_n = static_cast<int>(med);
+  }
+  return true;
 }
 
 bool load_layout(const std::string& path, LayoutState& out) {