feat: a signal switched off switches its dependent math channels off too

A derived channel cannot be computed from a signal nobody acquires: `Source` resolves to a
*channel*, and a disabled signal has none -- so a channel that reads one silently computed
nothing, which is what "all math signals depending on it do not compute anymore" looked like
from outside.

Rather than leave it enabled and blank, the channels that read the signal follow it off:
`disable_dependent_math_channels()` walks the graphs to a **fixpoint**, because one math
channel may name another, and the switch goes through the same path the pane's own toggle
uses -- `MathChannel::enabled` then `apply_math_channels()`, the single writer -- so the
pane's switches follow. Their traces leave the plots with the signal's, and the notice says
both halves.

The channels that do *not* read it are untouched, which is the assertion's other half:

    [enable-all-probe] switched 'sine' off: traces_left=0 (want 0)
        math_off=[math x2,math3] math_on=[math lp]
        notice=''sine' is off: 1 trace left the plots; math off: math x2 and 1 more'

`math x2` and `math3` both source `sine`; `math lp` sources `noise` and stays on. The probe's
switch step now runs late enough that a math probe armed behind it has installed its graphs,
which is why the earlier reading was empty -- `app.math` had nothing in it yet, not
"no dependants".

Re-enabling the signal does not switch them back on, for the same reason it does not restore
the traces: that is the user's decision, and the rule is written down in
invariants-math-channels.md.

src/ui.cpp

@@ -2534,6 +2534,42 @@ void remove_trace(App& app, int row, int slot) {
   app.rows->assign(row, slot, std::string(), std::string(), 0);
 }
 
+/// Disables every math channel that (transitively) reads `name`, and answers with their names.
+///
+/// A derived channel cannot be computed from a signal nobody acquires: `Source` resolves to a *channel*, and a
+/// disabled signal has none -- so a channel left enabled would silently compute nothing. The rule is that the
+/// channels which depend on a signal follow it off, which is also what the pane's own switch would do by hand.
+/// The dependency is transitive: one math channel may name another, so a channel reading a disabled one goes
+/// too, and the loop runs to a fixpoint for that.
+std::vector<std::string> disable_dependent_math_channels(App& app, const std::string& name) {
+  std::vector<std::string> off;
+  if (name.empty()) return off;
+  const auto math_index_of = [&app](const std::string& n) -> int {
+    for (std::size_t k = 0; k < app.math.size(); ++k) {
+      if (app.math[k].name == n) return static_cast<int>(k);
+    }
+    return -1;
+  };
+  for (bool moved = true; moved;) {
+    moved = false;
+    for (MathChannel& channel : app.math) {
+      if (!channel.enabled) continue;
+      for (const MathNode& node : channel.nodes) {
+        if (node.op != MathOp::Source) continue;
+        const int k = math_index_of(node.source);
+        const bool reads_it = node.source == name;
+        const bool reads_a_disabled_one = k >= 0 && !app.math[static_cast<std::size_t>(k)].enabled;
+        if (!reads_it && !reads_a_disabled_one) continue;
+        channel.enabled = false;
+        off.push_back(channel.name);
+        moved = true;
+        break;
+      }
+    }
+  }
+  return off;
+}
+
 /// Takes a signal off every plot, and says how many traces went.
 ///
 /// Switching a signal off means "do not acquire this", so a trace of it left on a plot draws nothing: a slot

src/ui_app.hpp

@@ -1251,6 +1251,7 @@ void on_fixed_range_apply(GtkButton*, gpointer user);
 void on_y_fixed_clicked(GtkButton* button, gpointer user);
 void remove_trace(App& app, int row, int slot);
 int remove_signal_from_plots(App& app, const std::string& name);
+std::vector<std::string> disable_dependent_math_channels(App& app, const std::string& name);
 void on_remove_clicked(GtkButton* button, gpointer user);
 GtkWidget* menu_icon_button(const char* icon, const char* label);
 void on_row_menu_show_remove(GtkButton*, gpointer user);

src/ui_probe.cpp

@@ -1873,8 +1873,9 @@ gboolean enable_all_probe(gpointer user) {
                a->model->channel_count());
   std::fflush(stderr);
   // Then switch one off through its own switch in the signal list: a disabled signal must leave the plots,
-  // not sit there drawing nothing in a slot a live one could take.
-  g_timeout_add(250, +[](gpointer u) -> gboolean {
+  // not sit there drawing nothing in a slot a live one could take. Late enough that a math probe armed behind
+  // this one has had its graphs installed -- the dependants are the point of the second line.
+  g_timeout_add(2000, +[](gpointer u) -> gboolean {
     App* app = static_cast<App*>(u);
     if (app->model->channels().empty() || app->signal_list == nullptr) return G_SOURCE_REMOVE;
     const std::size_t signal = app->model->channels().front().signal;
@@ -1907,8 +1908,25 @@ gboolean enable_all_probe(gpointer user) {
         if (app->rows->row(r).signal_name[static_cast<std::size_t>(s)] == name) ++left;
       }
     }
-    std::fprintf(stderr, "[enable-all-probe] switched '%s' off: traces_left=%d (want 0) notice='%s'\n",
-                 name.c_str(), left, app->notice.c_str());
+    // Which *math* channels the disabled signal took with it: a derived channel cannot be computed from a
+    // signal nobody acquires, so the ones that read it follow it off -- and their traces go too.
+    std::string off_list;
+    for (const MathChannel& m : app->math) {
+      if (m.enabled) continue;
+      if (!off_list.empty()) off_list += ",";
+      off_list += m.name;
+    }
+    // Both halves: the channels that read it went off, and the ones that do not are still on.
+    std::string on_list;
+    for (const MathChannel& m : app->math) {
+      if (!m.enabled) continue;
+      if (!on_list.empty()) on_list += ",";
+      on_list += m.name;
+    }
+    std::fprintf(stderr,
+                 "[enable-all-probe] switched '%s' off: traces_left=%d (want 0) math_off=[%s] "
+                 "math_on=[%s] notice='%s'\n",
+                 name.c_str(), left, off_list.c_str(), on_list.c_str(), app->notice.c_str());
     std::fflush(stderr);
     return G_SOURCE_REMOVE;
   }, a);

src/ui_signals.cpp

@@ -27,13 +27,23 @@ void on_signal_switch(GObject* obj, GParamSpec*, gpointer user) {
   app->model->set_signal_enabled(index, on);
   if (!on) {
     // A trace of a signal nobody acquires draws nothing, and holds the slot a live one could take.
-    const int gone = remove_signal_from_plots(*app, name);
-    if (gone > 0) {
+    int gone = remove_signal_from_plots(*app, name);
+    // ... and the derived channels that read it cannot be computed at all, so they follow it off, and their
+    // traces leave with them.
+    const std::vector<std::string> off = disable_dependent_math_channels(*app, name);
+    for (const std::string& channel : off) gone += remove_signal_from_plots(*app, channel);
+    if (!off.empty()) apply_math_channels(*app);  // the single writer: the pane's switches follow it
+    if (gone > 0 || !off.empty()) {
       app->relayout_pending = true;
       rebuild_signal_panels(*app);  // the chips of the signals that are still plotted
-      set_notice(*app, "'" + name + "' is off: " + std::to_string(gone) +
-                           (gone == 1 ? " trace left the plots" : " traces left the plots"),
-                 4);
+      std::string what = "'" + name + "' is off: " + std::to_string(gone) +
+                         (gone == 1 ? " trace left the plots" : " traces left the plots");
+      if (!off.empty()) {
+        // The channel names already read as what they are ("math x2"), so the sentence does not prefix them.
+        what += "; math off: " + off.front() +
+                (off.size() == 1 ? "" : " and " + std::to_string(off.size() - 1) + " more");
+      }
+      set_notice(*app, what, 5);
     }
   }
   // The enable state is part of the session (a signal can be on only as a math channel's source).