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/acquisition.hpp

@@ -48,6 +48,20 @@ class Acquisition {
   bool capture_want() const noexcept {
     return capture_want_.load(std::memory_order_acquire);
   }
+
+  /// Asks for one **forced trigger acquisition** -- the trigger panel's Force button. The producer
+  /// consumes the request on its next packet and opens a capture at that packet's newest stamp,
+  /// which the engine's ordinary latch then publishes with the configured pre/post window. The
+  /// request is dropped rather than retried when there is nothing to force: trigger mode off, no
+  /// channel resolved, the engine disarmed, or a capture already filling (a capture in flight *is*
+  /// the acquisition being asked for).
+  ///
+  /// **Write it under `ScopeModel::mutex()`, together with the `armed` flag the click also sets.**
+  /// The producer holds that lock for a whole packet -- it reads the panel's `armed` at the top of
+  /// one and consumes this request at the bottom -- so a request written between the two would be
+  /// read by a packet whose engine was still disarmed, and then dropped.
+  void request_force() noexcept { force_want_.store(true, std::memory_order_release); }
+
   /// The capture, for the popover's row and the probe. Counters and errors are read
   /// cross-thread through PacketCapture's own published fields.
   const PacketCapture& capture() const noexcept { return capture_; }
@@ -110,6 +124,9 @@ class Acquisition {
   std::atomic<bool> paused_{false};
   PacketCapture capture_;
   std::atomic<bool> capture_want_{false};
+  /// The pending Force request (see `request_force()`): a flag and not a queue, because two clicks
+  /// inside one packet are one acquisition, which is what a momentary button means.
+  std::atomic<bool> force_want_{false};
   std::uint32_t capture_slots_ = 1000;
   /// Written once, by the GTK thread, before `capture_want_` is ever published true;
   /// the producer reads it only after the flag, so no lock is needed.