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.hpp

@@ -30,6 +30,10 @@ enum class MathOp {
 const char* math_op_name(MathOp op);
 int math_op_count();
 MathOp math_op_at(int index);
+/// An op's position in `math_op_at()`'s order. That order is the *dialog's* -- what a dropdown or a
+/// menu lists, and what a session file stores -- and it is deliberately not promised to be the
+/// enum's declaration order, so a caller that needs a position must ask rather than cast.
+int math_op_index(MathOp op);
 /// Inputs the op takes: 0 for `Source`, 1 for the unary ones, 2 for the arithmetic.
 int math_op_arity(MathOp op);
 /// Which of the per-node parameters an op actually reads, so a dialog can show only those rows --