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.

help/C/org.udpscope.app/trigger.page

@@ -133,9 +133,10 @@
 
     <p>Two things make it different from <gui>multiple events</gui> beside it, and the panel offers
     both because they answer different questions. <em>It never runs out of room</em>: each signal's
-    image is the size of the plot on screen, so a display of a thousand acquisitions costs what a
-    display of one costs — one image per signal, whatever the plot's trace slots — and there is no
-    count to choose. <em>And it fades</em>, in one of two ways — <gui>fades by</gui> picks them:</p>
+    image is sized to the plot on screen and capped, so a display of a thousand acquisitions costs
+    what a display of one costs — one image per signal, whatever the plot's trace slots, and a very
+    wide window does not make it grow — and there is no count to choose. <em>And it fades</em>, in
+    one of two ways — <gui>fades by</gui> picks them:</p>
     <list>
       <item><p><gui>acquisitions</gui>: every acquisition keeps
       <gui>per acquisition</gui> of what was there before it (0.5, the default, is the schedule a