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.cpp

@@ -1051,6 +1051,38 @@ void Acquisition::on_data(const udps_frame_t* frame) {
   }
   last_stamp_ = last_stamp_used;
 
+  // A Force pressed since the last packet: one acquisition, at the present moment, without waiting
+  // for the condition to hold. Read *here* -- the end of the packet, past the channel loop -- for
+  // the reason the latch itself is deferred out of that loop: `last_stamp_` is the newest stored
+  // sample by now, so the capture opens on the moment the user pressed, and every channel has
+  // pushed that sample, so the cut the latch goes on to make cannot clamp it away
+  // (`publish_event()` clamps to each value ring's `produced()`).
+  //
+  // It opens the capture and nothing else. The latch below -- reached through `engine.process()`
+  // by an edge, through `engine.force()` here -- is what cuts the window, bumps the latch count,
+  // sets the trigger time and applies the mode's re-arm rule, so a forced acquisition is framed,
+  // pinned and recorded exactly like one the condition produced. A capture already in flight is
+  // not cancelled: it *is* the acquisition being asked for, and the engine refuses the request
+  // rather than restarting it.
+  if (force_want_.exchange(false, std::memory_order_acq_rel)) {
+    if (engine.force(last_stamp_)) {
+      std::fprintf(stderr,
+                   "[udpscope] trigger: forced acquisition, edge at %.3f s -- want "
+                   "[-%.4f..%.4f] s, %.4f s of history behind it, mode=%s\n",
+                   last_stamp_, event_span - post_want, post_want, last_stamp_ - oldest_s,
+                   tc.mode == TriggerMode::Single ? "single (disarms at the latch)" : "normal");
+      std::fflush(stderr);
+    } else {
+      // Nothing to force. Every cause is a state the user can see from the panel -- a trigger that
+      // is off, a signal that is not acquired, the engine disarmed, a capture already filling --
+      // so it is named on stderr rather than swallowed.
+      std::fprintf(stderr,
+                   "[udpscope] trigger: force ignored (enabled=%d armed=%d ch=%d pending=%d)\n",
+                   int(tc.enabled), int(engine.armed()), trig_channel, int(engine.pending()));
+      std::fflush(stderr);
+    }
+  }
+
   model_.trigger_armed().store(engine.armed(), std::memory_order_relaxed);
   // The edge goes out before the flag that makes the UI read it (release/acquire on
   // `trigger_pending`), so a tick that sees a capture open sees which edge it is for. A