feat: the op is picked when a node is added, not changed afterwards

\`add node\` made a gain and nothing else, so every other op was two steps. It
is a menu of the twelve now, each carrying its index through
\`math_op_index()\`'s space rather than a cast of the enum -- which is also
what \`math_dialog_sync()\` now uses to drive the op dropdown, since that
order is the dialog's and not the declaration's.

src/math_channel.cpp

@@ -106,6 +106,13 @@ MathOp math_op_at(int index) {
   }
 }
 
+int math_op_index(MathOp op) {
+  for (int i = 0; i < math_op_count(); ++i) {
+    if (math_op_at(i) == op) return i;
+  }
+  return 0;
+}
+
 int math_op_arity(MathOp op) {
   switch (op) {
     case MathOp::Source: return 0;