feat: a backwards wire re-sorts the graph instead of being refused

The evaluator walks the node list in order, which used to make the order the
user's problem -- an input of an earlier node was not even drawn as
connectable. `math_connect()` runs Kahn's algorithm over the trial edge set,
with ties broken by the current index so a graph already in order is not
permuted, and remaps every index the dialog holds in one block. A self wire
and a cycle are refused with a message and change nothing.

src/math_channel.cpp

@@ -215,15 +215,15 @@ double math_step(const MathChannel& channel, MathRuntime& rt, const float* value
   return rts[static_cast<std::size_t>(out)].value;
 }
 
-bool math_channel_valid(const MathChannel& channel, std::string* why) {
-  const std::size_t n = channel.nodes.size();
+bool math_channel_valid_nodes(const std::vector<MathNode>& nodes, int output, std::string* why) {
+  const std::size_t n = nodes.size();
   const auto fail = [&](const std::string& text) {
     if (why != nullptr) *why = text;
     return false;
   };
   if (n == 0) return fail("empty graph");
   for (std::size_t i = 0; i < n; ++i) {
-    const MathNode& node = channel.nodes[i];
+    const MathNode& node = nodes[i];
     if (node.op == MathOp::Source && node.source.empty()) {
       return fail("node " + std::to_string(i + 1) + ": no channel chosen");
     }
@@ -238,10 +238,15 @@ bool math_channel_valid(const MathChannel& channel, std::string* why) {
       }
     }
   }
-  if (channel.output < 0 || static_cast<std::size_t>(channel.output) >= n) {
+  if (output < 0 || static_cast<std::size_t>(output) >= n) {
     return fail("no output chosen");
   }
+  if (why != nullptr) why->clear();
   return true;
 }
 
+bool math_channel_valid(const MathChannel& channel, std::string* why) {
+  return math_channel_valid_nodes(channel.nodes, channel.output, why);
+}
+
 }  // namespace udpscope