docs: help pages for cursors, measurements and math channels

src/help/09-cursors.txt

@@ -0,0 +1,33 @@
+<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>.

src/help/10-measurements.txt

@@ -0,0 +1,39 @@
+<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.

src/help/11-math.txt

@@ -0,0 +1,35 @@
+<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.

src/help/udpscope-help.gresource.xml

@@ -17,5 +17,8 @@
     <file>06-navigate.txt</file>
     <file>07-pause.txt</file>
     <file>08-trigger.txt</file>
+    <file>09-cursors.txt</file>
+    <file>10-measurements.txt</file>
+    <file>11-math.txt</file>
   </gresource>
 </gresources>

src/help_window.cpp

@@ -38,6 +38,9 @@ constexpr HelpPage kPages[] = {
     {"Viewing", "Navigating the plot", "/org/udpscope/help/06-navigate.txt"},
     {"Viewing", "Pause and play", "/org/udpscope/help/07-pause.txt"},
     {"Viewing", "Trigger", "/org/udpscope/help/08-trigger.txt"},
+    {"Analysis", "Cursors", "/org/udpscope/help/09-cursors.txt"},
+    {"Analysis", "Measurements", "/org/udpscope/help/10-measurements.txt"},
+    {"Analysis", "Math channels", "/org/udpscope/help/11-math.txt"},
 };
 constexpr int kPageCount = static_cast<int>(sizeof(kPages) / sizeof(kPages[0]));