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

src/help/06-navigate.txt

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+<b>Navigating the plot</b>
+
+The plot pans and zooms with the mouse and with the keyboard. Every gesture acts on the row the
+pointer is over, and because the rows share one time axis, an x change moves all of them.
+
+<b>How to reach it</b>
+
+<b>Wheel</b> zooms both axes · <b>Shift+wheel</b> zooms x only · <b>Ctrl+wheel</b> zooms y only.
+<b>Drag</b> pans · <b>Ctrl+drag</b> is a box zoom on x · <b>Alt+drag</b> a box zoom on y.
+<b>+</b> and <b>-</b> zoom both axes; <b>Ctrl</b> with them restricts to x, <b>Alt</b> to y.
+<b>Arrow keys</b> pan, <b>f</b> fits, <b>Escape</b> cancels a gesture in progress.
+The full table is in the shortcut overview (<tt>Ctrl+/</tt>).
+
+<b>Worth knowing</b>
+
+• The keyboard zoom's modifiers are <i>not</i> the wheel's, and that is deliberate: on common
+  layouts <tt>+</tt> <i>is</i> shift+<tt>=</tt>, so a bare zoom-in already arrives with Shift
+  set and Shift could not mean anything. Ctrl/Alt mirror the box-zoom drags instead.
+• A <i>pinned</i> view is navigable within its own data but cannot leave it. While an event is
+  held, a capture is filling, or the view is paused, a zoom <i>in</i> works normally — "not
+  outside the event" is not "not readable" — but a pan or a zoom out is clamped to the data.
+• A live view is clamped to the window you asked for, not to what the ring happens to hold.
+  That is why a buffer that has only just started filling leaves its left-hand side empty
+  instead of squeezing the window down to the first few samples.
+• Zooming hard into a held capture is exact. The renderer finds the visible samples by binary
+  search over the real timestamps, never over a nominal sample grid, so a capture whose clock
+  wandered does not drift away from its axis as you zoom.
+
+<b>Example</b>
+
+Pause a live view, then press <tt>Ctrl++</tt>: x narrows and y does not move. Press
+<tt>Alt++</tt>: y narrows and x does not. Eight doublings into a held 2 s capture, the trace
+still reaches both margins — the zoom probe reports <tt>gap_px</tt> and <tt>end_px</tt> within
+a pixel or two at every step.