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/cursors.page

@@ -63,6 +63,16 @@
       <item><p>A marker past the newest sample reads <code>--</code>. With only one marker
       over data the row reads <code>partial</code>: a difference between a number and nothing
       is not a number.</p></item>
+      <item><p>The hover crosshair also writes its two coordinates where the eye already
+      looks: the value where its horizontal line meets the y axis, and the time where its
+      vertical line meets the x axis — each in a small box among that axis' own tick labels,
+      in the axis' own format. Each box is centred on the line it names, and its background
+      covers what is behind it, so a tick label underneath never prints through the number.
+      The preference's <gui>Crosshair axis labels</gui> turns the pair off.</p></item>
+      <item><p>In a split row the horizontal line runs across <em>every</em> plot of the row,
+      not just the one under the pointer: the plots share one y range, so it is one value
+      along the whole row. It stops at the row's edges — another row has a scale of its
+      own.</p></item>
       <item><p>In <gui>mix</gui> each marker also places a crossing annotation per trace,
       which is what lets you see on the plot which sample a number came from. It is <em>on
       that sample</em>, so when the marker falls between two it sits a little off its own