fix: math in a file view -- the buffer was emptied, and the derived ring filled twice

Two defects, both reached by adding a math channel to an open waveform or a replayed
event, and both measured before and after.

**A file view has no producer, so a structural change left it empty.** `set_math_channels()`
rebuilds the channels *and* the buffer, and a rebuild drops the history: right for a stream,
whose producer refills it from the next packet, and wrong for a file, which has nothing to
refill it with. The `--open` dump read `0 samples vs 0 time slots` for *every* channel,
the file's own `sine`/`square`/`ramp`/`noise` included, though the load had reported 402 --
that is what "adding a math node breaks the plotter" was. `reload_file_view()` re-reads the
file after such a change, which is also what computes the derived channels over it: the
load's own pushes run the math once the set is in place, so `--open` with a session that
defines graphs works for the first time.

**The file path pushed into the derived rings, and so did the engine.** `push_loaded_slot()`
did not honour the rule `ChannelInfo::math` states for the packet path ("the ingest must skip
it"), so each loaded slot put a value in a derived ring and `push_math_sample()` then put the
computed one: two samples per slot, a trace at double rate -- the derived channels read
`804 samples vs 402 time slots`. Skipping them there is the whole fix.

Readings now, all equal, which is the assertion this file's own invariant makes:

    --open the file:  math x2  402 samples vs 402 time slots, y=[-1.9021, 0.0063]  (2x the sine)
                      math lp  402 vs 402, mean=-0.4900 (a real filter output, not zero)
    live:             math x2  25100 vs 25100
    replayed event:   math x2  4000 vs 4000

I first tried filtering the file's *derived columns* out by name, since an export writes
every plotted channel. It removed the double count and zeroed `math lp`, because the column
it dropped was the source a session graph reads -- so it is reverted: those columns are
legitimate data, and the fix belongs in the push, not the loader.

src/ui.cpp

@@ -4179,6 +4179,29 @@ gboolean on_app_key_pressed(GtkEventControllerKey*, guint keyval, guint, GdkModi
   return TRUE;
 }
 
+/// Re-reads the file a *file view* is showing; true when something was reloaded.
+///
+/// Any structural change -- a math channel added, a signal enabled, a plan change -- rebuilds the buffer
+/// and drops its history. That is right for a stream, whose producer refills it from the next packet. A
+/// file has no producer, so the same rebuild leaves *every* channel reading `0 samples` and the view is
+/// simply empty: "adding a math channel breaks the plot" was this, and it is the same emptiness whichever
+/// structural change caused it. The file's own path is the only thing that can refill it, so it is loaded
+/// again -- which is also what computes the derived channels over it, since the load's own pushes run the
+/// math once the set is in place.
+bool reload_file_view(App& app) {
+  if (!app.replaying && app.model->config().open_waveform.empty()) return false;
+  const std::string path =
+      !app.waveform_name.empty() ? app.waveform_name : app.model->config().open_waveform;
+  if (path.empty()) return false;
+  WaveformLoadReport report;
+  std::string error;
+  if (!load_waveform_into_model(*app.model, path, &report, &error)) {
+    set_notice(app, "cannot reload " + path + " after the change: " + error, 6);
+    return false;
+  }
+  return true;
+}
+
 /// The header bar carries only the controls you reach for *while watching a signal*;
 /// everything you set up once lives in the overflow menu. Nine text buttons in a
 /// header bar is a wall of words to read past every time you want the one you need,