trigger: a phosphor display keeps the last N acquisitions; and a Force button

Two changes in the trigger pane, landed together.

**Persistence.** Trigger mode keeps the last N published acquisitions and draws them fading toward
the background, N being a panel control (1..8, default 1) with N=1 meaning today's behaviour
exactly, so the knob is the switch. The layers are the accumulate path's own -- one layer per
(signal, age) in the row's free trace slots, named with the accumulate vocabulary so
`layer_base_name()` strips the suffix and `signal_color()` resolves the base signal -- so a layer is
not a signal: the signal list, the colours and the measurements see one row per signal.
`ScopeModel` retains the outgoing snapshot in seven atomic `shared_ptr` slots (no lock, no copy; the
snapshot already exists) and a layer's channel is resolved **by name inside the snapshot**, because
the channel list is renumbered by any structural change since that acquisition was cut.
`RowSet::sync_layers()` re-plans from the rows and the depth alone, so a latch costs no rebuild, and
`event_data_window()` skips layer slots so an older acquisition can never re-pin the axis. The
schedule is 0.5^age -- the acquisition on screen at full brightness and each older one half the
previous, independent of N, so raising the depth adds dimmer acquisitions rather than rescaling the
ones already on screen. Memory is bounded by the ring's own `--buffer-mb` rather than by the depth,
which is the bound that cannot be driven into an OOM, and the panel says when either bound bites.
This also wired up `set_trace_alpha()`, which the accumulate path was setting and nothing was
reading -- so the documented 0.35 translucency of an accumulation was opaque in fact.

**Force.** In the trigger pane beside Arm/Disarm, shown only while trigger mode is on: one
acquisition now, with the configured window. It is a synthetic edge rather than a second publisher
-- the engine opens one capture at the present moment and everything else is the ordinary latch, so
a forced acquisition is cut, pinned, framed, recorded and cached exactly like a latched one, and
the button owns no window and no publish. The pre-trigger gate is deliberately skipped (capture now
is a request for the history that exists, and the snapshot reports its own shorter pre_s), a
capture already filling is refused rather than restarted, and the state afterwards is the mode's:
Normal stays armed, Single disarms. The request is one atomic flag written under the model mutex
with `armed` -- the producer reads `armed` at the top of a packet and the request at the bottom, so
a request written between the two would be dropped -- and consumed at the end of a packet, after
the channel loop and before the pending publish.

**A defect fixed in passing, in the function Force was modelled on:** `on_arm_clicked()` passed
`TriggerConfig::channel` (an index into `channels()`) to `set_signal_enabled()` (an index into
`signals()`). With one signal acquired, arming switched on the stream's clock signal and left the
watched signal untouched -- and since a signal the panel was not showing is a change, it rebuilt
the buffer under the click, dropping the very pre-trigger window the arming, and now the Force, was
about to capture. Measured before: `0.0100 s of history behind it`; after: `2.5299 s`.
`trigger_signal_index()` is now the one resolution, used by both buttons.

src/ui/trigger-panel.ui

@@ -2,10 +2,11 @@
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
-  The trigger panel's frame: one labelled row per setting, the two buttons that run it, and the search
-  block. Its *heading* is not here: the heading row carries the switch that turns trigger mode on and
-  off, and this panel's body is what goes insensitive when it is off, so the heading has to be its
-  sibling rather than a child of it (`build_trigger_panel()` returns the two together).
+  The trigger panel's frame: one labelled row per setting, the pair of buttons that run it
+  (Arm/Disarm and Force), and the search block. Its *heading* is not here: the heading row carries the
+  switch that turns trigger mode on and off, and this panel's body is what goes insensitive when it is
+  off, so the heading has to be its sibling rather than a child of it (`build_trigger_panel()` returns
+  the two together).
 
   Every row here is a label and a place for a control, and the control itself is built in code. A
   dropdown's model and a spin button's range are the *application's* (the condition list, the level
@@ -208,8 +209,34 @@
       </object>
     </child>
     <child>
-      <object class="GtkButton" id="trigger_arm">
-        <property name="label" translatable="yes">Arm</property>
+      <object class="GtkBox" id="persistence_row">
+        <property name="orientation">horizontal</property>
+        <property name="spacing">6</property>
+        <child>
+          <object class="GtkLabel">
+            <property name="label" translatable="yes">persistence</property>
+            <property name="halign">start</property>
+          </object>
+        </child>
+      </object>
+    </child>
+    <child>
+      <object class="GtkBox" id="arm_row">
+        <property name="orientation">horizontal</property>
+        <property name="spacing">6</property>
+        <child>
+          <object class="GtkButton" id="trigger_arm">
+            <property name="label" translatable="yes">Arm</property>
+            <property name="hexpand">1</property>
+          </object>
+        </child>
+        <child>
+          <object class="GtkButton" id="trigger_force">
+            <property name="label" translatable="yes">Force</property>
+            <property name="hexpand">1</property>
+            <property name="tooltip-text" translatable="yes">take one acquisition now, with the pre/post window above, without waiting for the condition</property>
+          </object>
+        </child>
       </object>
     </child>
     <child>