docs: help pages for cursors, measurements and math channels

src/help/10-measurements.txt

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+<b>Measurements</b>
+
+A measurement is a statistic of the <i>visible window</i> of one signal, recomputed as that
+window moves. The pane lists them with their values; each can also draw its result on the plot.
+
+<b>How to reach it</b>
+
+The measurements button in the toolbar, or <tt>m</tt>. The pane's <tt>+</tt> adds one; clicking
+a row opens the dialog where the signal, the type, the threshold share and the on-plot marks are
+set, with <b>Save</b> and <b>Delete</b>.
+
+<b>Worth knowing</b>
+
+• The types are average, top, bottom, min, max, peak to peak, amplitude, pulse duration, period
+  and frequency. <i>top</i> and <i>bottom</i> are the means of the upper and lower halves, which
+  is why a unit sine reads 0.637 (2/π) rather than 1.
+• The on-plot marks come in two kinds and the mix is the reading: a <i>level</i> mark is a ruler
+  at a value, a <i>time</i> mark is a marker at a position. An average draws one level mark; a
+  max draws a level mark <i>and</i> a time mark at the first sample that reached it.
+• Marks cannot be dragged. A measurement mark says what the measurement measured, so it is
+  created unselectable — as is the trigger marker.
+• A pass is throttled to a cadence rather than run every frame, so a ruler over a live view is
+  <i>stepped</i>. The number and the ruler come from the same pass, so they always agree.
+• An invalid pass keeps its <i>level</i> marks and drops its <i>time</i> marks. A level is a
+  property of the signal and outlives the window; a position inside the window does not. That is
+  what stops the rulers blinking off at a mode change.
+• A window with too few crossings reads <tt>invalid</tt> / <tt>--</tt> rather than a plausible
+  invented number. Frequency over a window holding one edge is the usual case.
+• <tt>frequency</tt> is reported in Hz and <tt>period</tt>/<tt>pulse duration</tt> in seconds;
+  every other type carries the channel's own unit.
+• Cost is bounded by reading the summary pyramid the producer already maintains, not the raw
+  window. The values are still exact: a bucket's sum is a sum.
+
+<b>Example</b>
+
+On a 1 Hz ±1 sine: <tt>average=0</tt>, <tt>top=0.636620</tt>, <tt>bottom=-0.636620</tt>,
+<tt>max=1</tt>, <tt>min=-1</tt>, <tt>peak to peak=2</tt>, <tt>amplitude=1</tt>,
+<tt>pulse duration=0.5 s</tt> (duty 0.5), <tt>period=1 s</tt>, <tt>frequency=1 Hz</tt>. Over a
+2 M-sample window those two passes read about 0.1% of the samples.