docs: help pages for export, sessions and the status bar

src/help/12-export.txt

@@ -0,0 +1,31 @@
+<b>Saving and export</b>
+
+Export writes the <i>visible range</i> of every plotted trace to a file: NumPy <tt>.npy</tt> or
+<tt>.csv</tt>, chosen by the extension you type.
+
+<b>How to reach it</b>
+
+<b>Export visible range…</b> in the overflow menu, or <tt>Ctrl+E</tt>. A file chooser asks where;
+the format follows the path's extension.
+
+<b>Worth knowing</b>
+
+• The table is collected at the moment you click, not when the chooser returns. A live view keeps
+  scrolling while a dialog is up, so a late collection would write a window you never saw.
+• The rows are the <b>stored samples</b>, never the LOD buckets the plot draws from. A row count
+  far below the span times the stored rate means something cut the table from the pyramid.
+• The stored rate is not always the wire rate. A window long enough to need decimation is stored
+  averaged at a lower rate, and the export carries <i>that</i> — read the status bar's ⓘ popover
+  for the figure before trusting a sample count.
+• Two time columns. <tt>time</tt> is the display timeline (relative to the trigger edge while an
+  event is held) and <tt>stream_time</tt> is the source's own clock. Both are strictly increasing
+  and evenly spaced; duplicate or backwards stamps would mean the file held two clocks.
+• An export taken <i>mid-capture</i> writes the partly filled window that is on screen, which is
+  shorter than the event span. Export outside a capture if you want the whole event.
+
+<b>Example</b>
+
+A 2 s window at 10 kS/s over 3 traces writes 20001 rows and 5 columns
+(<tt>time</tt>, <tt>stream_time</tt>, three signals). The <tt>.npy</tt> is
+<tt>64·⌈(10+len(header)+1)/64⌉ + rows·(16 + 4·cols)</tt> bytes. Read it back with
+<tt>pd.DataFrame(np.load(path))</tt>.

src/help/13-sessions.txt

@@ -0,0 +1,34 @@
+<b>Sessions and settings</b>
+
+udpscope stores one session file per source, so reopening a source gives you the scope you
+closed. There is no global preferences file: the session <i>is</i> the settings.
+
+<b>How to reach it</b>
+
+Nothing to open — the file is written as you work. Its path is in the About dialog, and the
+directory is <tt>$XDG_CONFIG_HOME/udpscope</tt>, or <tt>~/.config/udpscope</tt> when that is
+unset. Each file is named <tt>&lt;host&gt;_&lt;port&gt;.toml</tt>.
+
+<b>Worth knowing</b>
+
+• What a session persists: the time window, the rows and each row's slot signals, the
+  calibration, which signals are enabled, the math channels with their graphs, and the
+  measurements pane.
+• Signal identity is the <i>name</i> throughout, because a new CONFIG renumbers the channels.
+• A file written by a run against a source whose streamer was down would name no signals at all,
+  so such a run does <b>not</b> write: a configured session is never overwritten by a run that
+  configured nothing.
+• The multicast group and the interface live <i>inside</i> the file, because its name cannot
+  carry them. A file from an older build has neither key and loads as unicast on the default
+  route.
+• The startup chooser lists only files that name at least one signal, which is what keeps it the
+  sources you configured rather than every port you have ever tested against.
+• Deleting a file is a supported way to start a source over; the chooser's <tt>×</tt> does
+  exactly that.
+
+<b>Example</b>
+
+Connect to <tt>127.0.0.1:8000</tt>, drop three signals on row 1 and set a 2 s window.
+<tt>~/.config/udpscope/127.0.0.1_8000.toml</tt> now holds <tt>rows=1 window_s=2</tt> with
+<tt>signals = ["sine", "ramp", "noise", ""]</tt>, and the next run reports
+<tt>layout loaded from …/127.0.0.1_8000.toml</tt> with those signals already acquired.

src/help/14-status.txt

@@ -0,0 +1,36 @@
+<b>Status bar and diagnostics</b>
+
+The status bar is two tiers. The bar itself is one glance line — the state, the rate and the
+window — and everything else is behind the ⓘ button at its right-hand end.
+
+<b>How to reach it</b>
+
+The bar is always visible under the plot. Click <b>ⓘ</b> for the detail popover.
+
+<b>Worth knowing</b>
+
+• The badge is one glyph in three colours: lit and dim alternating means data is arriving, and a
+  steady idle colour means it is not. A badge that never blinks is not a working badge.
+• The state field is one word from a fixed set: <tt>live</tt>, <tt>paused</tt>, <tt>armed</tt>,
+  <tt>capturing</tt>, <tt>event held</tt>, <tt>disarmed</tt>. In trigger mode the bar adds
+  exactly one more field, how long ago the last event was.
+• Nothing on the bar may change width with the stream, because the bar sits directly above the
+  plot and a wider label would relayout it. That is why the rate collapses to one figure or a
+  range rather than listing per channel, and why an elapsed time steps to a coarser unit instead
+  of growing digits.
+• <tt>gaps</tt> counts packets the receiver knows it missed. It is the number that matters: a
+  non-zero count means samples are absent from the buffer, not merely late.
+• <tt>reconnects</tt> counts sessions re-established with the source. Over unicast a climbing
+  count usually means a second client is taking the source's single session.
+• The starvation notice — <i>connected, but no new frames in the last second</i> — is that same
+  situation seen from the other side. Pointing the app at a port where nothing answers produces
+  it too.
+• <tt>stored</tt> versus <tt>wire</tt> in the popover is the decimation. They differ exactly
+  when the window is too long to store at full rate.
+
+<b>Example</b>
+
+The bar reads <tt>● live · 10.0 kS/s · 2 s window</tt>. Behind ⓘ:
+<tt>Stream  connected, configured / packets 717 · frames 716 · gaps 0 / 3 signals · 3 channels</tt>,
+<tt>Buffer  200.0 MB · 1677 s span · time x1 / stored 10.0 kS/s (wire 10.0 kS/s)</tt>,
+<tt>Plot 1  3 traces · 195000 samples</tt>, <tt>Reconnects  0</tt>.

src/help/udpscope-help.gresource.xml

@@ -20,5 +20,8 @@
     <file>09-cursors.txt</file>
     <file>10-measurements.txt</file>
     <file>11-math.txt</file>
+    <file>12-export.txt</file>
+    <file>13-sessions.txt</file>
+    <file>14-status.txt</file>
   </gresource>
 </gresources>

src/help_window.cpp

@@ -41,6 +41,9 @@ constexpr HelpPage kPages[] = {
     {"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"},
+    {"Output and state", "Saving and export", "/org/udpscope/help/12-export.txt"},
+    {"Output and state", "Sessions and settings", "/org/udpscope/help/13-sessions.txt"},
+    {"Output and state", "Status bar and diagnostics", "/org/udpscope/help/14-status.txt"},
 };
 constexpr int kPageCount = static_cast<int>(sizeof(kPages) / sizeof(kPages[0]));