ui: a measurement's noise band and HF reject are settable and persisted

Two rows in the measure dialog, built the way the threshold and side rows are and appended
beside them: the band as a percentage of the window's own peak-to-peak (the unit the engine
takes), the corner in Hz with 0 off. They are seeded when editing, shown only where they do
something -- the band for the crossing types, since it moves the edge decision and nothing
else, the reject for all of them, since it sits before both walks -- and read into
`MeasureOptions` on apply.

They persist as two *named* keys, `noise` and `hf`, on the same `o <<` chain as the keys
beside them and read back through the same keyed reader, so a session written before this
change loads with 10.0 and 0.0: today's behaviour exactly, with no version check.

[configs] now proves the round trip rather than assuming it. The probe sets the first
measurement's band and corner to 7.5 and 250 before saving, so a key that never reached the
file reads back as the defaults:

    [configs] round trip read=1 comment='written by the configs probe' ... measures=4 math=0 noise=7.5 hf=250

The rest of that line is unchanged, and the run is free of Gtk-CRITICAL.

src/ui_app.hpp

@@ -910,6 +910,12 @@ struct MeasureDialog {
   GtkWidget* threshold_label = nullptr;
   GtkWidget* side_row = nullptr;
   GtkWidget* side_label = nullptr;
+  /// The two noise-rejection rows. `noise` is a percentage of the window's peak-to-peak, `hf` a corner in
+  /// Hz with 0 off; both are shown only where they do something (see `measure_dialog_sync`).
+  GtkWidget* noise = nullptr;
+  GtkWidget* noise_row = nullptr;
+  GtkWidget* hf = nullptr;
+  GtkWidget* hf_row = nullptr;
   /// The names the signal dropdown was built from, in order. `apply` resolves an index
   /// through *this*, not through the model's signal list: the dropdown lists the plotted
   /// traces, so index `i` here is the i-th trace -- indexing the CONFIG with it selected