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_probe.cpp

@@ -2953,6 +2953,13 @@ gboolean configs_probe(gpointer user) {
   std::fprintf(stderr, "[configs] before=%s active=%s\n", joined().c_str(),
                active_configuration(a->source_host, a->source_port).c_str());
 
+  // A non-default band and corner before the save, so the round trip below proves the two keys are
+  // *written*: left at their defaults, a key that never made it to the file would read back the same.
+  const bool has_measure = !a->measures.empty();
+  if (has_measure) {
+    a->measures.front().options.noise_pct = 7.5;
+    a->measures.front().options.hf_fc = 250.0;
+  }
   const bool saved = save_configuration_as(*a, "probe", "written by the configs probe");
   std::fprintf(stderr, "[configs] save-as=%d names=%s active=%s\n", saved ? 1 : 0,
                joined().c_str(), active_configuration(a->source_host, a->source_port).c_str());
@@ -2967,11 +2974,14 @@ gboolean configs_probe(gpointer user) {
       if (!sig.empty()) ++signals;
     }
   }
+  const double noise_back = st.measures.empty() ? -1.0 : st.measures.front().options.noise_pct;
+  const double hf_back = st.measures.empty() ? -1.0 : st.measures.front().options.hf_fc;
   std::fprintf(stderr,
                "[configs] round trip read=%d comment='%s' buffer_mb=%.6g relative=%d "
-               "capture=%u plots=%zu signals=%d measures=%zu math=%zu\n",
+               "capture=%u plots=%zu signals=%d measures=%zu math=%zu noise=%.6g hf=%.6g\n",
                read ? 1 : 0, st.comment.c_str(), st.buffer_mb, st.relative_time ? 1 : 0,
-               st.capture_packets, st.rows.size(), signals, st.measures.size(), st.math.size());
+               st.capture_packets, st.rows.size(), signals, st.measures.size(), st.math.size(),
+               noise_back, hf_back);
 
   forget_configuration(a->source_host, a->source_port, "probe");
   std::fprintf(stderr, "[configs] after forget names=%s\n", joined().c_str());