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

src/help/12-export.txt

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+<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>.