scope: full-screen figures, second windows, gated measurements, per-configuration events

A batch of view and instrument work, with the probes and docs for each.

- **Full screen per figure row**: a toggle on each row's top-right corner, revealed by
  pointer proximity (`kMaximizeRevealPx` around its placed rect, per row, read from an
  overlay-level motion controller so a revealed button under the pointer cannot flicker);
  the plot menu offers the same as *Full screen this figure* / *Restore the layout*. The
  arrangement is saved and restored through `RowState`, so nothing plotted moves. A layout
  of one figure carries no toggle.
- **Second window**: `open_scope_window()` copies a figure (cells, width shares, signals in
  their colours) or opens an empty one, with its own `RowSet` and `PlotArea`; the main
  window's tick binds, acquires and repaints it, mirroring the time window and pin. Entries:
  the plot menu's *Open this plot in a new window* and the overflow menu's *New scope window*.
- **Layout editor from the plot**: the plot menu gains *Edit the layout...* / *Done editing
  the layout*, flipping the app's one toggle.
- **Measurements gate**: `MeasureOptions::gate` restricts a measurement to the held/pause
  snapshot (live falls back to the live ring), persisted with the session and shown in the
  measure dialog.
- **Crosshair axis badges**: a preference (`crosshair_axis_labels`, on by default) and the
  plotter's badges where the lines meet the axes, in the axes' own formats; the plot menu
  and the probe read them.
- **Event cache per configuration**: `EventStore` is keyed `<source>/<configuration>/`,
  `retarget_event_store()` follows every configuration switch, and a pre-configuration cache
  (loose files) is adopted into the configuration's folder on first start.
- **GTK4 lifecycle**: the main window's handlers move from the non-existent `"destroy"`
  signal to `close-request`; scope windows are closed with it and the application quits
  explicitly (the exit run's hang). The exit probe closes with `gtk_window_close()`.
- Probes: `[maximize-probe]` (single-figure rule and the corner reveal), `[scope-probe]`,
  `[split-probe]` cell crosshair leg, `[prefs-probe]` store keying, `[configs]` store key,
  `[rowmenu]` layout-editor entry; docs, help pages, README and AGENT.md updated.

Also fixes a probe-only use-after-free: `toggle_maximize_figure_row` replaces the figure
object, so the area must be re-pointed (the button's path does; the probe's raw call did not).

help/C/org.udpscope.app/layout.page

@@ -29,7 +29,8 @@
         add a trace.</p>
       </item>
       <item>
-        <p>Press <key>l</key>, or pick <gui>Layout editor</gui> from the overflow menu, to add
+        <p>Press <key>l</key>, pick <gui>Layout editor</gui> from the overflow menu, or pick
+        <gui>Edit the layout…</gui> from a plot's right-click menu, to add
         and remove rows: a <gui>×</gui> appears at the top left of each plot and a <gui>+</gui>
         on each boundary between rows, including above the first and below the last.</p>
       </item>
@@ -58,12 +59,33 @@
     <p>All the plots of a row, and of the whole figure, share one time window: zooming or
     panning along time in any of them moves all of them, and the vertical cursors are drawn in
     every plot. The plots of a split row also share one y range: the y labels are drawn at the
-    left of the first plot only, the horizontal cursors sit at the same values in every plot, and a
+    left of the first plot only, the horizontal cursors sit at the same values in every plot, the
+    hover crosshair's horizontal line spans all of them, and a
     zoom, pan, fit or fixed range in any of them applies to the whole row. The time labels
     sit under the bottom row, and under any row above it whose columns differ from the row below. A row holds at most 4 plots and the figure at most 8
     in total; the <gui>+</gui> buttons go grey at the limit.</p>
   </section>
 
+  <section id="fullscreen">
+    <title>A row full screen, and a second window</title>
+    <list>
+      <item><p>Every row carries a <gui>full-screen</gui> button in its top-right corner: it comes
+      out of hiding when the pointer moves near that corner and hides again when it leaves, so the
+      plot is not decorated by a control you are not reaching for. One click makes that row fill the plot area — the
+      other rows step aside with everything they were plotting — and the same button (now the
+      restore icon) brings the whole layout back exactly as it was. It is a view, not an edit:
+      nothing is saved, nothing is dropped, and the rows' traces, colours and cursors come back
+      where they were. The plot's right-click menu offers the same as <gui>Full screen this
+      figure</gui> (and <gui>Restore the layout</gui> while one is enlarged). A layout of a single
+      figure has nothing to enlarge *to*, so it carries no button.</p></item>
+      <item><p>A plot's right-click menu has <gui>Open this plot in a new window</gui>: a second
+      window showing the same stream through a copy of that figure (its cells, their widths and
+      every trace), with its own axes following the main window's time window. The overflow menu's
+      <gui>New scope window</gui> opens the same kind of window with an empty figure. Closing one
+      closes that view only; the main window keeps acquiring.</p></item>
+    </list>
+  </section>
+
   <section id="knowing">
     <title>Worth knowing</title>
     <list>