fix: only a window change sets the time scale; a fit states the whole of it

The time scale is more than the axis: it is also the trigger's event span, the
buffer partition and the control's readout. A zoom or a pan was already clean --
the controller writes the member axis and `clamp_to_data()` caps the span -- but
both fits moved `window_s_` alone and only restated the control. The event span
then stayed on the previous window until the next decade step or unit change
applied the fitted value for real, so one action changed the time scale twice.

`apply_window_scale()` is split out of `apply_time_window()` for the fits to use:
it moves the partition, the event span, the control and the session, and leaves
the view alone, because `set_time_window()` re-arms the live follow and would
throw a held event or a paused view back onto the stream. The fits report the
span they framed, `fit_all()` delegates its x half to `fit_x()` so the two cannot
drift, and the probe drives the menu handlers rather than `RowSet` so it sees the
half that lives there. `[fit-probe]` gained `event_span=`, which must equal
`window=` on every line.

Also moves the plotter pin to 03c41ad (three build warnings, no behaviour).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/help/06-navigate.txt

@@ -19,6 +19,10 @@ The full table is in the shortcut overview (<tt>Ctrl+/</tt>).
 • 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 gesture never changes the <i>time window</i>, only the view inside it. The window is what the
+  header control states, and it also sets the trigger's event span and how the buffer is stored —
+  so a wheel that redefined it would re-plan the buffer and drop your history. The two fits
+  <i>do</i> set it, deliberately, and the control then reads the fitted value back.
 • 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.