fix: a repaired source is reconciled above the validation line, not below it

`math_dialog_sync_form()` wrote the label and then moved a node whose source
names an unacquired channel -- a mutation that changes what
`math_dialog_problem()` reports. With the sync re-entry now guarded, the nested
call that used to repair the label is suppressed, so the line kept describing a
source that had just been fixed and left `Apply` insensitive until the next
mutation. The reconcile moves above the write and the dropdown reuses its list.

docs/claude/invariants-math-channels.md

@@ -25,7 +25,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   two-click protocol is gone), drag a box to move it; **wheel zooms about the pointer, background
   drag pans, `f` fits, `Ctrl+0` resets**.
   Its widgets are created once and only shown, hidden and set by `math_dialog_sync()`, so no rebuild
-  can invalidate a live callback handle -- the same shape the measurements dialog uses. `UDPSCOPE_PROBE_MATH_MS` defines two graphs and opens the editor (a construction crash test
+  can invalidate a live callback handle -- the same shape the measurements dialog uses.
+  `UDPSCOPE_PROBE_MATH_MS` defines two graphs and opens the editor (a construction crash test
   as much as a state check), and the dump prints each derived channel's extremes beside the
   source's. Read it on the fast mock, where the two assertions that matter are **`gaps=0`** (the
   math is on the producer thread, so what it can break is packet capture, never the frame rate)

src/ui.cpp

@@ -4085,9 +4085,9 @@ gboolean math_canvas_key(GtkEventControllerKey*, guint keyval, guint, GdkModifie
   return FALSE;
 }
 
-/// Adds a node of `op` and selects it. The grid placement is the same one the auto-place uses, so a
+/// Adds a node of `op` and selects it, at the first grid cell whose box overlaps nothing, so a node
-/// node added here lands clear of the others -- 160 is narrower than `kNodeW` plus a wire's tread,
+/// added here lands clear of the others -- 160 is narrower than `kNodeW` plus a wire's tread, which
-/// which is why the tidy layout uses a wider pitch; this is only a first guess at a free cell.
+/// is why the tidy layout uses a wider pitch.
 void math_add_node(MathDialog* d, MathOp op) {
   // A node added to a graph that was *linear* -- its output was the last node -- becomes the output,
   // which is what "add a gain and it applies" means. A graph whose output is an earlier node is a
@@ -4233,6 +4233,20 @@ void on_math_op_changed(GObject* object, GParamSpec*, gpointer user) {
 /// nodes its inputs are wired to. Called only through `math_dialog_sync()`, which guards it against
 /// the re-entry the dropdowns' `notify::selected` would otherwise cause.
 void math_dialog_sync_form(MathDialog* d) {
+  // A source naming a channel nobody acquires is moved to the first acquired one -- see the dropdown
+  // below for why. It happens *here*, above the validation line, because the move is a mutation that
+  // changes what `math_dialog_problem()` reports: reconciled after the label is written, a repaired
+  // source would be described as broken, with `Apply` insensitive, until some later sync.
+  std::vector<std::string> names;
+  const bool have_form =
+      d->selected >= 0 && d->selected < static_cast<int>(d->nodes.size());
+  if (d->source_drop != nullptr && have_form) {
+    names = math_source_names(*d->app);
+    MathNode& sel = d->nodes[static_cast<std::size_t>(d->selected)];
+    if (!names.empty() && std::find(names.begin(), names.end(), sel.source) == names.end()) {
+      sel.source = names.front();
+    }
+  }
   // Before the early return below: an out-of-range selection is exactly the state in which a stale
   // validation line is most misleading -- a graph with no node selected can still be broken, and the
   // user has no form to read instead.
@@ -4275,22 +4289,15 @@ void math_dialog_sync_form(MathDialog* d) {
   if (d->source_drop != nullptr) {
     // The *acquired* channels, which is what Apply validates against. Listing every signal let a
     // user pick a source that could never be made valid -- and put the stream's timestamp signal,
-    // which has no samples at all, at the top of the list.
+    // which has no samples at all, at the top of the list. A source that is not in this list would
-    const std::vector<std::string> names = math_source_names(*d->app);
+    // read NaN for ever while the dropdown showed the first entry as if it were selected, so the
+    // node was already moved to the first entry at the top of this function.
     GtkStringList* list = gtk_string_list_new(nullptr);
     guint pick = 0;
-    bool found = false;
     for (std::size_t i = 0; i < names.size(); ++i) {
       gtk_string_list_append(list, names[i].c_str());
-      if (names[i] == n.source) {
+      if (names[i] == n.source) pick = static_cast<guint>(i);
-        pick = static_cast<guint>(i);
-        found = true;
-      }
     }
-    // A source that is not in the list is a graph naming a channel nobody acquires -- it would read
-    // NaN for ever while the dropdown showed the first entry as if it were selected. The node is
-    // moved to what the form is actually showing, so what is on screen is what the graph says.
-    if (!found && !names.empty()) n.source = names.front();
     gtk_drop_down_set_model(GTK_DROP_DOWN(d->source_drop), G_LIST_MODEL(list));
     gtk_drop_down_set_selected(GTK_DROP_DOWN(d->source_drop), pick);
     g_object_unref(list);