feat: the export writes an MDF 3.30 file, so a window loads in ScryLab

A third format beside `.npy` and `.csv`, written by hand in `src/export.cpp` --
MF4 is HDF5 and would mean a library, MF3 is a handful of fixed blocks. The
layout is the asammdf 8.8.27 reader's own field list, byte-diffed against a
known-good 3.30 file it wrote; the shape is the one ScryLab's data-source
contract asks for: one channel group, record-interleaved, the master as the x
axis, and the unit in the channel's conversion block.

So in MDF the master is `abs_seconds`, named `time` -- the clock that is the
same in every display mode -- and the plotted axis goes out as an ordinary
channel, `time_relative`. `ExportTable::Column` gained the CONFIG's unit, which
only this writer spells out; the `.npy`/`.csv` bytes are unchanged, and
`write_export()` now picks the writer for both the dialog and the probe.

src/ui.cpp

@@ -2371,10 +2371,10 @@ void on_export_response(GObject* source, GAsyncResult* result, gpointer user) {
   }
 
   // The extension picks the writer, so the filter the user chose is honoured by way of
-  // the name it put in the entry rather than by a second piece of state.
-  const bool csv = path.size() >= 4 && path.compare(path.size() - 4, 4, ".csv") == 0;
+  // the name it put in the entry rather than by a second piece of state. `.npy` is what an
+  // unknown extension gets, which is the dialog's default filter too.
   std::string error;
-  const bool ok = csv ? write_csv(path, ctx->table, error) : write_npy(path, ctx->table, error);
+  const bool ok = write_export(path, ctx->table, error);
   if (!ok) {
     set_notice(*ctx->app, "export failed: " + error);
     return;
@@ -2411,10 +2411,18 @@ void on_export_clicked(GtkButton*, gpointer user) {
   gtk_file_filter_set_name(csv, "CSV (*.csv)");
   gtk_file_filter_add_pattern(csv, "*.csv");
   g_list_store_append(filters, csv);
+  // MDF 3.30, which is what ScryLab (and every MDF reader) opens: offered by both of the
+  // extensions the format is stored under.
+  GtkFileFilter* mdf = gtk_file_filter_new();
+  gtk_file_filter_set_name(mdf, "MDF 3.30 (*.mf3, *.mdf)");
+  gtk_file_filter_add_pattern(mdf, "*.mf3");
+  gtk_file_filter_add_pattern(mdf, "*.mdf");
+  g_list_store_append(filters, mdf);
   gtk_file_dialog_set_filters(dialog, G_LIST_MODEL(filters));
   gtk_file_dialog_set_default_filter(dialog, npy);
   g_object_unref(npy);
   g_object_unref(csv);
+  g_object_unref(mdf);
   g_object_unref(filters);
 
   gtk_file_dialog_save(dialog, GTK_WINDOW(app->window), nullptr, on_export_response,