fix: persistence is the trace's own phosphor, not a density behind a stroke

The plotter now deposits every acquisition -- the newest included -- into one grid per trace and
draws no solid trace over it, each grid in that trace's colour and with a one-sweep ceiling (its own
commit, **1b8b55d**). This side needed almost no change of its own, because the colour was already
the trace's and the trace's colour is `RowSet::signal_color()` -- the invariants' one route -- but the
figures it reads back are now per trace:

- **The ceiling is left at the library's default of one sweep**, no longer derived from the fade.
  Deriving it from the steady state is the gain offset the user reported between the latest
  acquisition and the persistent image; a signal crossing the same place every acquisition now settles
  at the trace's colour, at full strength, and the newest sweep reads as the trace it is a picture of.
- **`cells`/`grid_bytes` are the sum over the row's traces** -- one grid each, a trace with no series
  holding none -- and both the probe and the exit dump carry `traces=` beside `rows=`. That is what
  makes the per-signal memory figure read: **556 537 cells and 2.2 MB per signal**, at 8904600 bytes
  over the four one-signal rows of the probe's run.

The probes, re-read on this build (the `verification.md` command, `--rate 500000 --batch 1000`):

    [density-probe] off enabled=0 ... rows=0 traces=0 cells=0 ink=0.0 peak=0.0000
    [density-probe] on enabled=1 ... rows=4 traces=4 cells=2226150 ink=226786.0 peak=1.0000
    [density-probe] step-acquisitions ... peak=1.5000
    [density-probe] time-before ... peak=2.1359
    [density-probe] step-time ... ratio=0.367879
    [density-probe] frame cost: tick 0.62 -> 6.96 ms (184/23 frames), draw 2.02 -> 6.16 ms (177/18)
    [udpscope]   density: frames=748 rows=4 traces=4 cells=2226150 grid_bytes=8904600 ink=489400.2 peak=2.2 upload_bytes=2226150

`on` `peak=1.0000`, `step-acquisitions` `peak=1.5000` and `step-time` `ratio=0.367879` are the legs
that were expected, unchanged; the *draw* mean is now **below** the run before the split (2.02 -> 6.16
against 2.09 -> 20.91), because the solid traces are no longer drawn over the image.

Docs updated with it: the trigger manual page (`persistence` is the trace, one image per signal, the
newest included), the README's persistence section, `invariants-trigger-and-pinning.md` (five rules,
the ceiling and the no-stroke rule among them), `verification.md` and `probe-readings.md` (the lines
above), the trigger panel's comment and tooltips, and the plotter submodule.

`cmake --build build --clean-first -j4`: **0 errors, the documented three warnings** (two
`MathNode::source`, one `-Woverlength-strings` at 12352); `make -C build test-ui` **13 ok**;
`make -C build test-help` **green** (15 pages); the self-check **0 `Gtk-WARNING`/`Gtk-CRITICAL`** and
`gtk warnings: 0`, two `buffer allocated:` lines.

src/ui_probe.cpp

@@ -6102,10 +6102,12 @@ double density_probe_tau() {
   return s > 0.0 ? std::min(s, 3600.0) : 1.0;
 }
 
-/// What the density holds over every row it is on: the rows, the cells, and the two numbers the
-/// plot reports. A row with the display off contributes nothing -- it has no grid at all.
+/// What the density holds over every row it is on: the rows, the traces (one grid each), the cells,
+/// and the two numbers the plot reports. A row with the display off contributes nothing -- it has no
+/// grid at all.
 struct DensityReading {
   int rows = 0;
+  int traces = 0;
   unsigned long long cells = 0;
   double ink = 0.0;
   double peak = 0.0;
@@ -6116,11 +6118,17 @@ DensityReading density_read(const App& a) {
   for (int i = 0; i < a.rows->row_count(); ++i) {
     const RowSet::Row& row = a.rows->row(i);
     if (row.plot == nullptr || !row.plot->density().enabled) continue;
-    const plot::DensityGrid& g = row.plot->density_grid();
     ++r.rows;
-    r.cells += static_cast<unsigned long long>(g.cell_count());
-    r.ink += g.ink();
-    r.peak = std::max(r.peak, static_cast<double>(g.peak()));
+    // **One grid per trace**, each in its trace's own colour, so the row's cells/ink are the sum
+    // over its traces and the peak the brightest of them.
+    for (plot::u32 t = 0; t < row.plot->trace_count(); ++t) {
+      const plot::DensityGrid& g = row.plot->density_grid(t);
+      if (g.cell_count() == 0u) continue;  // an unbound slot holds no grid
+      ++r.traces;
+      r.cells += static_cast<unsigned long long>(g.cell_count());
+      r.ink += g.ink();
+      r.peak = std::max(r.peak, static_cast<double>(g.peak()));
+    }
   }
   return r;
 }
@@ -6129,12 +6137,18 @@ DensityReading density_read(const App& a) {
 /// plotter's own accumulation (its per-acquisition fade happens inside it, before the deposit), and
 /// the time step is `decay_density(tau)` -- the same call the frame tick makes with a frame's `dt`.
 double density_step(App& a, bool by_time) {
-  double before = 0.0;
-  for (int i = 0; i < a.rows->row_count(); ++i) {
-    const RowSet::Row& row = a.rows->row(i);
-    if (row.plot == nullptr || !row.plot->density().enabled) continue;
-    before += row.plot->density_grid().ink();
-  }
+  // The ink summed over every trace's grid (one grid per trace): the same total `density_read()`
+  // reports, so a step's ratio is off one reading.
+  const auto ink = [&]() {
+    double total = 0.0;
+    for (int i = 0; i < a.rows->row_count(); ++i) {
+      const RowSet::Row& row = a.rows->row(i);
+      if (row.plot == nullptr || !row.plot->density().enabled) continue;
+      for (plot::u32 t = 0; t < row.plot->trace_count(); ++t) total += row.plot->density_grid(t).ink();
+    }
+    return total;
+  };
+  const double before = ink();
   for (int i = 0; i < a.rows->row_count(); ++i) {
     plot::Plot* p = a.rows->row(i).plot;
     if (p == nullptr || !p->density().enabled) continue;
@@ -6144,12 +6158,7 @@ double density_step(App& a, bool by_time) {
       p->accumulate_density();
     }
   }
-  double after = 0.0;
-  for (int i = 0; i < a.rows->row_count(); ++i) {
-    const RowSet::Row& row = a.rows->row(i);
-    if (row.plot == nullptr || !row.plot->density().enabled) continue;
-    after += row.plot->density_grid().ink();
-  }
+  const double after = ink();
   return before > 0.0 ? after / before : 0.0;
 }
 
@@ -6180,11 +6189,11 @@ void density_probe_report(App& a, const char* what, double ratio) {
     tc = a.model->trigger();
   }
   std::fprintf(stderr,
-               "[density-probe] %s enabled=%d fade=%s tau=%.3f factor=%.3f rows=%d cells=%llu "
-               "ink=%.1f peak=%.4f ratio=%.6f\n",
+               "[density-probe] %s enabled=%d fade=%s tau=%.3f factor=%.3f rows=%d traces=%d "
+               "cells=%llu ink=%.1f peak=%.4f ratio=%.6f\n",
                what, tc.density ? 1 : 0,
                tc.density_decay == PersistDecay::Time ? "time" : "acquisitions", tc.density_tau_s,
-               tc.density_factor, r.rows, r.cells, r.ink, r.peak, ratio);
+               tc.density_factor, r.rows, r.traces, r.cells, r.ink, r.peak, ratio);
   std::fflush(stderr);
 }