docs: help pages for the time window, navigation, pause and trigger

src/help/07-pause.txt

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+<b>Pause and play</b>
+
+Pause freezes the <i>display</i>, not the acquisition. The producer keeps filling the ring at
+full rate; the view binds an immutable snapshot of the window that was on screen, and the axis
+is pinned to it.
+
+<b>How to reach it</b>
+
+The pause button in the toolbar, or <tt>Space</tt>. Press it again to resume, which rebinds the
+live ring and puts the window back on the newest data.
+
+<b>Worth knowing</b>
+
+• Nothing is lost while paused. Because the producer never stops, resuming shows current data
+  with no hole in the history behind it.
+• The axis is pinned to the <i>snapshot's own stamps</i>, not to the window that was requested.
+  A window reaching back past the oldest retained sample yields a shorter snapshot, and the
+  pinned axis is that shorter span — so you see a narrower view rather than an empty margin.
+• A paused view is pinned exactly as a held trigger event is: zoom in freely, but a pan or a
+  zoom out is clamped to the snapshot's data.
+• Pausing is the easiest way to measure a layout or read a cursor precisely, because nothing
+  under the pointer moves between frames.
+
+<b>Example</b>
+
+Pause with a 2 s window over a ring holding 1.4 s: the frozen axis reads
+<tt>[-1.40 .. 0.00]</tt> rather than <tt>[-2.00 .. 0.00]</tt> with 0.6 s of blank. The pause
+probe reports <tt>ring_produced</tt> still growing across the pause and
+<tt>event_size</tt> non-zero exactly while paused.