fixed plot flicker, measurement cost and toolbar icons

The flicker ("no data" at no constant frequency, worse with the measurements
and cursors panes up) was a torn read of the window's first stamp: acquire()
pins the window to the ring's oldest slot, and reading it at paint time races
the producer lapping it, returning a stamp one ring span too new. x0() is the
origin every trace point is reconstructed from, so one torn read produced a
refused neighbours(), a filter that dropped three quarters of the points, and
a polyline shifted off its axis. It is cached by acquire() under the lock now.

EventSeries::neighbours() answered from the (x - x0)/dx grid, whose drift from
the real arrival stamps arrives as a constant offset in seconds -- a doubling
share of the screen per zoom step, which is the "zooming into a held event
clips it more from the left" report. Both series binary-search the stamps now.

The measurements pane went through the LOD pyramid rather than reading its
window raw (19.7 ms per recompute to 1.2 ms, 0.1% of a 2 M-sample window) and
recomputes on a cadence; the math engine lost a per-slot heap allocation, a
per-sample source lookup by name and a fetch_add that cost more than the
arithmetic it counted.

The cursors pane's mix mode drew a line per *named* slot, so a slot holding a
signal that can never produce a channel -- the stream's timestamp -- got a row
that could only read "--", and such names were re-saved every session. It
counts bound traces now and apply_layout() refuses them.

Four toolbar icons are drawn for this app and compiled in as a GResource: the
stock ones were a record dot for the trigger and three magnifiers for cursors,
measurements and math.

New probes and dump lines for all of it (trace ink, measure cost, math cost,
event zoom, toolbar icons); README and CLAUDE.md record what they cure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/icons/udpscope.gresource.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!--
+  The four icons this app had to draw for itself, compiled into the binary so there is
+  still exactly one file to run. `gtk_icon_theme_add_resource_path()` wants the tree an
+  icon theme has, so the prefix is followed by `<size>/<context>`; `scalable` is the one
+  that means "an SVG, use it at any size".
+-->
+<gresources>
+  <gresource prefix="/org/udpscope/icons">
+    <file>scalable/actions/udpscope-trigger-symbolic.svg</file>
+    <file>scalable/actions/udpscope-cursors-symbolic.svg</file>
+    <file>scalable/actions/udpscope-measure-symbolic.svg</file>
+    <file>scalable/actions/udpscope-math-symbolic.svg</file>
+  </gresource>
+</gresources>