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).

src/ui_status.cpp

@@ -56,6 +56,26 @@ const char* badge_markup(BadgePhase phase) {
   return "<span foreground=\"#5A5A5A\">\u25CF</span>";
 }
 
+/// What the trigger status circle is saying, by state: 0 solid green (an event is on screen,
+/// or a capture is filling), 1 and 2 the bright and dim halves of the waiting flash, 3 red
+/// (disarmed). Same glyph and the same colour family as the activity badge -- the two sit
+/// beside each other on the status row, and two alphabets there would be a puzzle -- and one
+/// glyph in every state, for the same width-locked reason.
+const char* trigger_circle_markup(int state) {
+  switch (state) {
+    case 0:
+    case 1:
+      return "<span foreground=\"#45E045\">\u25CF</span>";
+    case 2:
+      return "<span foreground=\"#1C5A1C\">\u25CF</span>";
+    case 3:
+      return "<span foreground=\"#E04545\">\u25CF</span>";
+    default:
+      break;
+  }
+  return "<span foreground=\"#5A5A5A\">\u25CF</span>";
+}
+
 /// A short-lived line in front of the status text: the outcome of an action that the
 /// picture does not show by itself (a drop that changed nothing, an export that wrote a
 /// file). `tick()` clears it when it expires.