measure: top/bottom/amplitude read the flat levels, not the extremes

A bench scope separates Vtop/Vbase from Vmax/Vmin because a band-limited
edge overshoots and rings: the mock's `square` at 30 harmonics reaches
±1.179 where its plateau is ±1. On the rippled square the old readings were
a property of the edges rather than of the waveform -- `top` 0.986496,
`amplitude` 1.179350 -- and `amplitude` reported the overshoot.

`top`, `bottom` and `amplitude` are now the flat levels and their delta.
A flat level is a density question -- the plateau holds most of the
samples and the ringing a few percent -- so `measure_scan_buckets()` folds
a 64-bin histogram of the whole buckets' midpoints while it has the list in
hand (one observation per bucket, each bin carrying its buckets' exact sum
and finite count), and `measure_flat_levels()` takes, either side of the
split, the densest bin holding at least half the densest one's buckets --
the outermost of those, which is what makes a waveform with no plateau read
its peak -- and the exact mean of the samples of the buckets within 5 % of
the window's peak-to-peak of it. 5 % is deliberately under the ~9 %-of-swing
Gibbs overshoot. `amplitude` is `top - bottom` (a scope's Vamplitude), not
the half-swing it was.

Measured, fast mock, 2 s window, harmonics 0 -> 30: top 1.000000 ->
0.999983, bottom -1.000000 -> -0.999983, amplitude 2.000000 -> 1.999966,
while `peak to peak` keeps the extremes (2.000000 -> 2.358701). On the 1 Hz
sine top is 0.957988 (it was 0.636620, the half-wave mean; a sine has no
plateau, so the densest band is at its peak) with sd 0.0014 over a scrolling
window, and `peak to peak` is still exactly 2.

No raw pass and no new fetch: the histogram is the same fetched bucket list
read a second time, gated on a pane that holds one of the three types.
`raw` is unchanged (0.1% clean, 2.1% with the --noise 0.1 interferer,
against 0.1%/2.1% before) for 0.44 -> 0.50 ms and 0.52 -> 0.65 ms a pass.
A window the pyramid cannot summarize -- an hf-filtered walk, or one shorter
than a bucket -- keeps the old readings: the mean of each side of the split,
and the window's peak to peak for amplitude.

Docs: the flat-level invariant (and the two 2/pi assertions it replaces),
the probe readings, the manual's measurements page and the README.

src/ui_probe.cpp

@@ -1176,7 +1176,8 @@ gboolean measure_probe(gpointer user) {
   // The signal this probe measures is enabled *here*, for the same reason the noise probe enables its own:
   // a probe must not depend on another probe's side effects. `enable_all_probe` ends by switching a signal
   // off (it reads what a disabled signal does to the plots), and with `sine` gone the readings below were
-  // taken on `square` -- where `top` is 1.0, not the 2/pi the documented baseline is.
+  // taken on `square` -- where `top` is its plateau's 1.0 rather than the sine's flat level, and the numbers
+  // the docs quote are the sine's.
   {
     const std::vector<SignalInfo>& signals = a->model->signals();
     for (std::size_t i = 0; i < signals.size(); ++i) {