fix: bound the density grid, and make [notrace] a reading instead of a flood

The plotter caps a density grid at kDensityCellCap now (deps/plotter), so a 2 s
window with persistence on no longer scales the image with the window: the
probe's four one-signal rows read 1 044 714 cells / 4 178 856 bytes where they
read 2 226 150 / 8 904 600, and a moved frame uploads 1 044 714 bytes where it
uploaded 2 226 150. The probe's own legs are unchanged (on peak=1.0000,
step-acquisitions peak=1.5000, step-time ratio=0.367879).

The paint probe no longer counts a frame the density is drawing as *traceless*.
`Renderer::draw_traces()` returns while the ink is the image, so every frame of
a persistence display stages no trace: the dump read `traceless paints: 653
(637 after the first bind)` for a run whose real figure is 4 (0), and the
`[notrace]` line printed for each of those frames. It is now behind
UDPSCOPE_PROBE_NOTRACE -- a per-paint line is a log flood and not a reading --
while the counters it details stay in every build, and the line stays reachable
for the state it was written for (a bound row that draws nothing).

Docs corrected where they stated the old rule: the README, the trigger
invariants, the density probe's readings, the verification list (the new
variable) and the manual page behind the panel's tooltip.

src/layout_store.hpp

@@ -102,8 +102,8 @@ struct LayoutState {
     /// for: it is what the display does with the acquisitions, and there is no second switch.
     ///
     /// This is the **multiple event** display -- N retained acquisitions, drawn as N traces. The
-    /// phosphor density beside it is a different display with a different cost (one screen-sized
-    /// image against N window copies), which is why it has its own four fields below and why this
+    /// phosphor density beside it is a different display with a different cost (one bounded image a
+    /// trace against N window copies), which is why it has its own four fields below and why this
     /// one keeps the key it has always had: an older file's `trigger_persistence` is still the
     /// multiple-event depth.
     int persistence = 1;