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>

tools/mock_udpstreamer.py

@@ -16,6 +16,7 @@ Everything is float64 and unquantised.
 
 import argparse
 import math
+import random
 import socket
 import struct
 import time
@@ -87,6 +88,11 @@ def main():
                          "clock advances ten times slower than the wire delivers samples")
     ap.add_argument("--declared-rate", type=float, default=-1.0,
                     help="sampling_rate to put in CONFIG; -1 = the real rate, 0 = 'unknown'")
+    ap.add_argument("--clock-jitter", type=float, default=0.0,
+                    help="per-packet stamp jitter, as a fraction of one packet's span: a device "
+                         "that stamps each batch from its own clock does not land on a perfect "
+                         "grid, so the stream's timestamps wobble about one. Non-cumulative "
+                         "(the mean rate is unchanged); 0 = the ideal grid")
     args = ap.parse_args()
 
     dt = args.stream_scale / args.rate          # stream seconds per sample
@@ -134,9 +140,17 @@ def main():
             time.sleep(min(next_send - now, 0.002))
             continue
 
-        sock.sendto(build_data(counter, t_stream, dt, args.batch, args.freq), peer)
+        # The jitter varies each *packet's* own spacing rather than offsetting its start, because
+        # the stream's timestamps have to stay strictly increasing -- a jittered t0 can overlap the
+        # packet before it, and the buffer, the trigger and every binary search here assume a
+        # monotonic clock. The mean spacing is still `dt`, so the declared rate stays honest; what
+        # moves is the *local* spacing, which is what a uniform grid over the window cannot model.
+        pdt = dt
+        if args.clock_jitter > 0.0:
+            pdt = dt * (1.0 + random.uniform(-args.clock_jitter, args.clock_jitter))
+        sock.sendto(build_data(counter, t_stream, pdt, args.batch, args.freq), peer)
         counter += 1
-        t_stream += args.batch * dt
+        t_stream += args.batch * pdt
         next_send += period
         if args.duration > 0.0 and started is not None and now - started > args.duration:
             print("mock udpstreamer: done (%d packets)" % counter, flush=True)