docs: help pages for cursors, measurements and math channels

src/help/09-cursors.txt

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+<b>Cursors</b>
+
+Cursors read values off the traces. There are two kinds: one <i>shared vertical pair</i> that
+every row carries at the same time, and <i>horizontal rulers</i>, one pair per row, that belong
+to that row alone.
+
+<b>How to reach it</b>
+
+The cursors button in the toolbar, or <tt>c</tt>. The pane below the plot shows the readings and
+owns a <b>Reset</b> button. Drag a marker to move it. The mode dropdown offers four modes:
+<tt>vertical</tt> (the shared time pair), <tt>horizontal</tt> (the per-row rulers),
+<tt>both</tt>, and <tt>mix</tt>.
+
+<b>Worth knowing</b>
+
+• The vertical pair is <i>one</i> time for the whole figure, so moving it on any row moves it on
+  all of them. That is what makes <tt>dt</tt> meaningful across rows.
+• A ruler belongs to its row and nothing shares it, so two rows' rulers are independent.
+• <tt>mix</tt> is the reading mode: one line per <i>bound</i> trace, with the trace's own values
+  at each marker and their difference. A slot with a name but no channel behind it is not a
+  trace and gets no line.
+• A marker past the newest sample reads <tt>--</tt>. With only one marker over data the row
+  reads <tt>partial</tt>: a difference between a number and nothing is not a number.
+• In <tt>mix</tt> each marker also places a crossing annotation per trace, which is what lets
+  you see on the plot which sample a number came from.
+• The time row exists only in the modes that show the vertical pair. <tt>horizontal</tt> has no
+  time row, by design.
+
+<b>Example</b>
+
+Three traces in <tt>mix</tt> mode with both markers over data: 3 lines and 6 crossing marks,
+each line reading like <tt>sine  0.31 V  -0.84 V  -1.15 V</tt>. Move the second marker past the
+newest sample and the third column reads <tt>--</tt>.