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.

docs/claude/probe-readings.md

@@ -851,9 +851,13 @@ popover row reads `failed: <strerror>` — one stderr line, no retry storm again
 [defaults] calibrated: 2.5 -1.25 model=2.5 -1.25
 [configs] before=default active=default
 [configs] save-as=1 names=probe,default active=probe
-[configs] round trip read=1 comment='written by the configs probe' buffer_mb=8 relative=0 capture=0 plots=1 signals=4 measures=0 math=0
+[configs] round trip read=1 comment='written by the configs probe' buffer_mb=8 relative=0 capture=0 plots=1 signals=4 measures=0 math=0 noise=-1 hf=-1
 [configs] after forget names=default
 ```
+`noise=` and `hf=` are the first measurement's band and corner as they came back from the file. The probe
+sets them to **7.5 and 250** before saving, so a key that never reached the file reads back as the defaults
+(10 and 0) instead -- which is the only way a round trip can tell a written key from an absent one. They read
+`-1` when the saved configuration holds no measurement, which is what the run above does.
 
 | field | value it must read | the defect a wrong reading names |
 | --- | --- | --- |