docs: help pages for cursors, measurements and math channels

src/help/11-math.txt

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+<b>Math channels</b>
+
+A math channel is a signal computed from other signals. You build it as a small graph — source
+nodes, operations, one output — and udpscope then treats the result as an ordinary channel.
+
+<b>How to reach it</b>
+
+The math button in the toolbar, or <tt>g</tt>. The pane's <tt>+</tt> adds a channel; opening one
+shows the graph editor: nodes as boxes, click a box to select it and edit its parameters beside
+the canvas, click an output port then an input port to wire them, drag a box to move it. The
+orange dot marks the output node.
+
+<b>Worth knowing</b>
+
+• The graph is evaluated as an ordered list, so a node may only read ones <i>above</i> it. A
+  cycle is impossible by construction rather than rejected by a check.
+• A derived channel <i>is</i> a channel. It appears in the signal panel, can be plotted,
+  measured, exported, and used as a source for another math channel.
+• The math runs on the <b>stored</b> samples — decimated and averaged — so a cutoff you type in
+  Hz means the rate the filter will actually run at. Change the window across a decimation
+  boundary and the coefficients are rebuilt for the new rate.
+• A source may only name an <i>acquired</i> channel. The stream's timestamp signal is not one:
+  a graph sourced from it validates and then reads NaN for ever, so the dropdown does not
+  offer it.
+• The output defaults to the <i>last</i> node, and a node added to a linear graph becomes the
+  output. Choosing an earlier node explicitly is respected.
+• Every numeric field starts at its own identity value — gain 1, offset 0 — not at the bottom
+  of its range.
+
+<b>Example</b>
+
+<tt>[sine] → [gain x2]</tt> gives a channel reading <tt>y=[-2, 2]</tt>. A 100 Hz low pass on the
+same source passes 50 Hz at 0.97 and 2 kHz at 0.002; a 1 kHz band pass with Q=2 passes 200 Hz at
+0.05, 1 kHz at 0.95 and 4 kHz at 0.05. Each derived channel must hold exactly as many samples as
+the shared clock has time slots.