trigger: a phosphor persistence, driven by the plotter's density image

The trigger pane gains the display the user asked for and the library now provides, and the
persistence it already had is renamed to what it is.

**Persistence is one image; the last-N-traces display is "multiple events".** Both live in a new
`Advanced` expander in the trigger panel, together with the settings that are set up once and left
alone. `persistence` (a switch) turns on the plotter's `Plot::set_density()` for every row:
every triggered acquisition is accumulated into a screen-sized intensity grid drawn behind the
traces, brightness being how many acquisitions covered a cell -- no depth to choose, no memory
that grows with the acquisitions (2.2 MB over four 573x412 rows in the probe's run). `fades by`
picks the fade the user asked for: **acquisitions** (each keeps a factor of what was before it,
0.5 by default -- the phosphor afterglow) or **time** (a time constant in stream seconds, so the
display fades while you watch whatever the trigger rate).

- **Trigger mode only, as asked.** `apply_density()` (the tick) states the display on for `Held`
  and `Capturing` and off for `Live`, `Waiting`, `Paused` and a file, and turning it off drops the
  grid at once rather than at an acquisition a quiet trigger never sends. It is unpickable and
  reserves no layout space, exactly as the invalid-span bands are: a density can never be in the
  way of a cursor or a drop, and it cannot move the framing.
- **One deposit per acquisition, one frame after the frame that frames it.** `Plot::accumulate_density()`
  deposits what the plot holds through the transforms of its *last* layout -- the frame's own is
  computed inside the paint that follows the tick -- so `density_pending` is set beside
  `record_pending` and consumed by the next tick. In the same tick it was right on a Held-to-Held
  transition and wrong on the first latch, whose axis was still the live ring's. Switching the
  display on under an acquisition already on screen deposits it too, so the switch does not appear
  to do nothing until the next latch (`single` mode may never send one).
- **The session carries it**: `trigger_density`, `trigger_density_decay` ("acquisitions"/"time",
  the words the panel shows), `trigger_density_tau` and `trigger_density_factor`, written whether
  or not it is on -- and `trigger_persistence` still means the multiple-event depth, so an older
  file is unchanged.
- **The library changed twice under this**: the deposit now counts a cell **once per trace per
  sweep** (counting the hits put one dense sweep at the ceiling of the brightness scale and hid the
  sweeps behind it -- the probe's `on` leg read `peak=18` for a single sweep and reads `peak=1`
  now), and a frame's upload is every row's rather than the last row's. Both are in the plotter's
  own commit.

`UDPSCOPE_PROBE_DENSITY_MS` reads it all back off the plots -- the plotter's own `ink()`/`peak()` --
with the panel's own widgets driving the switch and the fade, in four legs: **off** (`rows=0
ink=0.0 peak=0.0000`), **on** (`rows=4`, `peak=1.0000`), **step-acquisitions** (`peak=1.0000 ->
1.5000`, the factor then the new sweep) and **step-time** (`ratio=0.367879`, `1/e` for `dt = tau`).
It also reads the frame cost as an A/B **inside one run** (the accumulators at the switch against
the frames after it): `tick 0.74 -> 9.99 ms`, `draw 2.09 -> 20.91 ms` at four rows and 2.2 M cells
on llvmpipe.

Acceptance: clean build 0 errors and exactly the three documented warnings (12352-byte GResource
blob, two `MathNode::source`), `test-ui` 13/13, `test-help` green, a plain run with **0
Gtk-CRITICAL / Gtk-WARNING** and `gtk warnings: 0`, and the panel's own frame reading
`boxes=16 controls_inside=15/15 advanced_widgets=5/5 advanced_inside=5/5` (the block's controls
counted from the expander's child, which is not in the widget-parent chain -- and the search
block's walk now names *its* expander instead of "the first one", which the second expander would
have stolen).

Docs: the trigger/pinning invariants carry the rule and its five consequences, `probe-readings.md`
the four legs and the dump line, `verification.md` the probe's sequence and assertions, and the
manual's trigger page explains both displays to a user.

src/layout_store.cpp

@@ -520,6 +520,14 @@ bool load_layout(const std::string& path, LayoutState& out) {
         double n = 0.0;
         num(n);
         t.persistence = static_cast<int>(n);
+      } else if (k == "density") {
+        t.density = v == "true";
+      } else if (k == "density_decay") {
+        t.density_decay = v == "time" ? "time" : "acquisitions";
+      } else if (k == "density_tau") {
+        num(t.density_tau_s);
+      } else if (k == "density_factor") {
+        num(t.density_factor);
       }
     };
     if (section == Section::Top) {
@@ -723,6 +731,13 @@ bool save_layout(const std::string& path, const LayoutState& state) {
   o << "trigger_band = " << state.trigger.noise_band << "\n";
   o << "trigger_hf = " << state.trigger.hf_fc << "\n";
   o << "trigger_persistence = " << state.trigger.persistence << "\n";
+  // The phosphor density, the display that is an image rather than a set of traces. Its four fields
+  // are written whether or not it is on, like every other trigger setting: a display set up once and
+  // switched off is still the display you come back to.
+  o << "trigger_density = " << (state.trigger.density ? "true" : "false") << "\n";
+  o << "trigger_density_decay = " << toml_escape(state.trigger.density_decay) << "\n";
+  o << "trigger_density_tau = " << state.trigger.density_tau_s << "\n";
+  o << "trigger_density_factor = " << state.trigger.density_factor << "\n";
   if (!state.comment.empty()) o << "comment = " << toml_escape(state.comment) << "\n";
   if (state.buffer_mb > 0.0) o << "buffer_mb = " << state.buffer_mb << "\n";
   o << "relative_time = " << (state.relative_time ? "true" : "false") << "\n";