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/measurements.page

@@ -80,8 +80,19 @@
       shorter than a level's own resolution, or one read through <gui>hf</gui> — where
       <gui>top</gui>/<gui>bottom</gui> fall back to the mean of each side of the split and
       <gui>amplitude</gui> to the window's peak to peak.</p></item>
+      <item><p>Each row names the <em>channel</em> it measures: its number among the active
+      channels (1., 2., … the same number the signal's badge in the bar carries), its name, and
+      the type — <code>1. sine · average</code>. The measurement dialog's signal dropdown
+      numbers its entries the same way. A channel list that changes (a signal switched off, a
+      new one enabled) renumbers, because the number is the answer to "which active channel is
+      this".</p></item>
       <item><p>The on-plot marks come in two kinds and the mix is the reading: a <em>level</em>
-      mark is a ruler at a value, a <em>time</em> mark is a marker at a position. An average
+      mark is a ruler at a value — its value written in a small box on the ruler, the way the
+      axis writes its own ticks — and a <em>time</em> mark is a marker at a position, with the
+      position's own badge beside it. A level ruler's badge carries its number; a time
+      marker's carries its time. Marks whose badges meet share one panel with one line each,
+      so a column of close readings is one boxed readout rather than a smear of texts; and a
+      box over a noisy trace is what keeps the number readable. An average
       draws one level mark; a max draws a level mark <em>and</em> a time mark at the first
       sample that reached it; <gui>peak to peak</gui> draws a ruler at each extreme and
       <gui>amplitude</gui> a ruler at each of its two flat levels — the pair is the span, so
@@ -136,6 +147,25 @@
     </list>
   </section>
 
+  <section id="gate">
+    <title>The gate</title>
+    <p>A measurement's dialog carries <gui>Gate enabled</gui>. Every measurement
+    with it set reads only the samples inside one shared band — violet, labelled <em>gate</em>,
+    carried by its own two draggable edges on the plots, which are not the cursors (those keep
+    meaning what the cursors pane reads). Measurements without it are untouched: they read the
+    whole window and share the walks they always shared.</p>
+    <p>The band is shown exactly while some measurement is gated — gate one and it appears,
+    gate or delete the last one and it goes — because it is one area that all of the gated
+    measurements share, not a property of any one of them. Drag an edge to resize it, grab the
+    middle to move it.</p>
+    <p>The band lives in the same relative time the axis labels use, so it stays where it was
+    put in both modes: live, a band at 300 ms to 100 ms ago stays put on screen while the data
+    streams beneath it — the gated read of the newest data a bench scope gives — and on a held
+    event it names a fixed slice of the frozen frame. A band with no samples in it — dragged
+    outside the view, or over a gap — reads <code>--</code> like any empty window. The band's
+    position and each measurement's gate flag are saved with the session.</p>
+  </section>
+
   <section id="example">
     <title>Example</title>
     <p>On a 1 Hz ±1 sine: <code>average=0</code>, <code>top=0.957988</code>,