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/event_series.hpp

@@ -7,6 +7,7 @@
 
 #include <algorithm>
 #include <memory>
+#include <string>
 #include <vector>
 
 namespace udpscope {
@@ -63,8 +64,30 @@ struct EventSnapshot {
   /// already O(n), and the same channel on two rows shares it instead of building and
   /// storing it twice.
   std::vector<plot::data::MinMaxPyramid> pyramids;
+  /// The channel *names* `values` is indexed by, so a snapshot can be re-bound after the channel
+  /// list was rebuilt -- and renumbered -- under it: a persistence layer names its signal and
+  /// resolves that name here, exactly as the trigger engine names its channel and for the same
+  /// reason (an index is a position in a list that any enable, disable or CONFIG renumbers).
+  /// Empty on a snapshot nobody re-binds later (a pause view).
+  std::vector<std::string> channel_names;
 };
 
+/// Bytes one snapshot holds: both stamp arrays, every channel's samples, the pyramids built for
+/// them and the names above. The number the persistence retention is measured in -- a retained
+/// snapshot is a whole window copied out of the ring, so what bounds it is bytes, not a count.
+inline std::size_t snapshot_bytes(const EventSnapshot& s) {
+  std::size_t bytes = s.stamps.size() * sizeof(f64) + s.abs_stamps.size() * sizeof(f64) +
+                      s.y.size() * sizeof(plot::Range) +
+                      s.channel_names.size() * sizeof(std::string);
+  for (const std::vector<f32>& v : s.values) bytes += v.size() * sizeof(f32);
+  // A pyramid entry is 32 bytes (`MinMaxPyramid::Entry`), one per level-`l` bucket of
+  // `kBucket << l` samples -- the whole index is about one byte per sample.
+  for (const plot::data::MinMaxPyramid& p : s.pyramids) {
+    for (u32 l = 0; l < p.level_count(); ++l) bytes += p.level_size(l) * 32u;
+  }
+  return bytes;
+}
+
 /// A plot::data::Series view of one channel of an EventSnapshot. The stamps are
 /// near-uniform (the arrival clock), so dx is the mean spacing and x_at returns
 /// the exact stamp; gather decimates by 2^level like the ring series does.