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_prefs.cpp

@@ -39,6 +39,7 @@ Settings read_rows(const PrefsForm& f) {
   s.relative_time = adw_switch_row_get_active(f.relative_row) != FALSE;
   s.memory_limit_mb = static_cast<int>(adw_spin_row_get_value(f.memory_row));
   s.theme = theme_name(adw_combo_row_get_selected(f.theme_row));
+  s.crosshair_axis_labels = adw_switch_row_get_active(f.crosshair_row) != FALSE;
   // The row shows the *effective* directory, so the default is written out in full rather than
   // described. Leaving it at that default is not a choice to pin it: the file keeps "unset", and the
   // setting goes on following the cache directory it is named after.
@@ -67,6 +68,7 @@ void fill_rows(PrefsForm& f, const Settings& s) {
   adw_switch_row_set_active(f.relative_row, s.relative_time ? TRUE : FALSE);
   adw_spin_row_set_value(f.memory_row, s.memory_limit_mb);
   adw_combo_row_set_selected(f.theme_row, s.theme == "light" ? 1 : (s.theme == "dark" ? 2 : 0));
+  adw_switch_row_set_active(f.crosshair_row, s.crosshair_axis_labels ? TRUE : FALSE);
   f.filling = false;
 }
 
@@ -146,6 +148,7 @@ void on_closed(AdwDialog*, gpointer user) {
   f.relative_row = nullptr;
   f.memory_row = nullptr;
   f.theme_row = nullptr;
+  f.crosshair_row = nullptr;
   f.browse = nullptr;
   f.parent = nullptr;
 }
@@ -160,13 +163,15 @@ void resolve_rows(PrefsForm& f) {
   f.relative_row = ADW_SWITCH_ROW(gtk_builder_get_object(f.builder, "relative_row"));
   f.theme_row = ADW_COMBO_ROW(gtk_builder_get_object(f.builder, "theme_row"));
   f.memory_row = ADW_SPIN_ROW(gtk_builder_get_object(f.builder, "memory_row"));
+  f.crosshair_row = ADW_SWITCH_ROW(gtk_builder_get_object(f.builder, "crosshair_axis_labels_row"));
   if (f.dir_row == nullptr || f.keep_row == nullptr || f.relative_row == nullptr ||
-      f.theme_row == nullptr || f.memory_row == nullptr) {
+      f.theme_row == nullptr || f.memory_row == nullptr || f.crosshair_row == nullptr) {
     f.dir_row = nullptr;
     f.keep_row = nullptr;
     f.relative_row = nullptr;
     f.theme_row = nullptr;
     f.memory_row = nullptr;
+    f.crosshair_row = nullptr;
     g_clear_object(&f.builder);
   }
 }
@@ -222,6 +227,7 @@ void prefs_open(PrefsForm& form, GtkWidget* parent, const Settings& current,
   g_signal_connect(form.relative_row, "notify::active", G_CALLBACK(on_row_notify), &form);
   g_signal_connect(form.memory_row, "notify::value", G_CALLBACK(on_row_notify), &form);
   g_signal_connect(form.theme_row, "notify::selected", G_CALLBACK(on_row_notify), &form);
+  g_signal_connect(form.crosshair_row, "notify::active", G_CALLBACK(on_row_notify), &form);
   if (GObject* reset = gtk_builder_get_object(form.builder, "reset_row"); reset != nullptr) {
     g_signal_connect(reset, "activated", G_CALLBACK(on_reset), &form);
   }
@@ -248,6 +254,7 @@ void prefs_probe_change(PrefsForm& form) {
   adw_switch_row_set_active(form.relative_row, TRUE);
   adw_spin_row_set_value(form.memory_row, 4096.0);
   adw_combo_row_set_selected(form.theme_row, 2);
+  adw_switch_row_set_active(form.crosshair_row, FALSE);
 }
 
 /// A changed memory limit reaches the running model. Only when it differs from the model's own (every
@@ -277,6 +284,9 @@ void open_preferences(App& app) {
   prefs_open(app.prefs, app.window, app.settings, [&app](const Settings& s, bool written) {
     app.settings = s;
     apply_memory_limit(app, s.memory_limit_mb);
+    // The crosshair's axis badges, at once like the theme: it is a drawing flag on the
+    // plots, and a change is the next frame's drawing.
+    app.rows->set_crosshair_axis_labels(s.crosshair_axis_labels);
     // Only a failure is announced. A change arrives once per keystroke in the directory row, and a
     // status line that flashed on each one would be the app shouting about its own bookkeeping.
     if (!written) set_notice(app, "could not write the preferences file", 4);