ui: the trigger panel offers a noise band and an HF reject

Two label-only rows in trigger-panel.ui (band_row, hf_row, after holdoff_row and before
mode_row) and the two spins the code appends into them: the band on the level spins' scale
in signal units, the HF reject at 0-1e7 Hz with 0 off. Both are published with the other
fields -- read before the model lock is taken, assigned inside it -- and both join the
value-changed loop, so editing either republishes the condition.

The rows are shown per type: the band for every type but Dropout, which is an absence a
band cannot describe, and the HF reject for all of them, since it sits before every
comparison. `sync_trigger_rows()` resolves a row as its spin's parent rather than keeping a
second handle to it: the row comes from the file and holds one child.

The exit dump counts the panel's controls, so it reads controls_inside=15/15 now -- and the
probe confirms it, along with both search readings and no Gtk-CRITICAL:

    [udpscope] trigger panel: boxes=15 controls_inside=15/15 search_widgets=4/4 search_inside=4/4

src/ui_app.hpp

@@ -170,6 +170,8 @@ struct App {
   GtkWidget* trigger_level_spin = nullptr;
   GtkWidget* trigger_pre_spin = nullptr;
   GtkWidget* trigger_holdoff_spin = nullptr;
+  GtkWidget* trigger_band_spin = nullptr;  ///< EventCondition::noise_band
+  GtkWidget* trigger_hf_spin = nullptr;    ///< EventCondition::hf_fc, 0 is off
   GtkWidget* signal_list = nullptr;
   GtkWidget* pause_button = nullptr;
   /// When the pause began, on the **monotonic wall clock**, or 0 while running. The elapsed time