test: the noise probe reads the trigger's band and HF reject

Six legs over one retained buffer, each a full rising-edge search through `BufferSearch` at
level 0, run back to back without yielding so every leg sees the same samples: the bare
comparator on a spiked sine, a 0.8 band, a 50 Hz reject, both, and the reject's group delay
on `square` -- which carries no impulses, so a one-pole's crossing of zero after a step is
exactly `tau*ln2` and the delay is an exact prediction rather than a coincidence.

    trigger band:  bare=49/2 band=7/2 ratio=7.00
    trigger hf:    hf=7/2 band+hf=7/2 band=7
    trigger delay: clean=4 filtered=4 delay=0.002206 want=0.002206 ratio=1.000

`ratio >= 5`, both HF legs within 2 of the band, `ratio` between 0.7 and 1.4, `state=2` on
every leg, no Gtk-CRITICAL. `bare` and `band` are proportional to the retained buffer, so
neither is a fixed number; the ratios and the agreement between legs are the assertions.

The probe enables the signals it measures itself rather than relying on another probe's side
effects -- `enable_all_probe` ends by switching one off again, which is how `sine` was missing
from the first run of this one -- and arms in two phases because that enable is a structural
change that replaces the store, so the ring has to fill before there is anything to search.
Its "no stream" line now names the channels that *are* there: an index alone says a probe
found nothing, not what it saw.

docs/claude/verification.md

@@ -522,3 +522,27 @@ hold ports.
 
 The probes are the regression net — when changing UI or plot-host behaviour, extend
 the probe/dump in `src/ui.cpp` rather than adding a separate harness.
+
+## The noise probe needs a spiked mock
+
+Every other probe runs against the plain mock. The noise probe reads what a *band* and an *HF reject* do to
+impulsive interference, so its mock must have some: an alternating-sign impulse train of 2 samples at
+amplitude 0.5 every 5 ms, added to `sine` alone (so a spiked channel can be read against a clean one in the
+same run).
+
+```bash
+cd /home/martino/Projects/tools/udpscope
+python3 tools/mock_udpstreamer.py --port 8108 --rate 1300000 --batch 1000 --duration 16 \
+  --spike-every 0.005 --spike-amp 0.5 --spike-width 2 > /tmp/noise-mock.log 2>&1 &
+sleep 1.5
+xvfb-run -a env UDPSCOPE_EXIT_AFTER_MS=14000 UDPSCOPE_ENABLE_ALL_MS=1000 \
+  UDPSCOPE_PROBE_NOISE_MS=6000 \
+  ./build/udpscope --server 127.0.0.1 --port 8108 2>&1 | grep -E 'noise-probe|CRITICAL'
+```
+
+The probe arms at 6 s rather than the 8 s a single-phase probe would use: its first phase enables the signals
+it measures and waits 2.5 s for the ring to refill after that structural change. `--spike-every`/`--spike-amp`/
+`--spike-width` are the mock's own; `--batch` must stay at or below 1000 (2000 samples x 5 signals x f64 is
+80 kB, past UDP's ~65507-byte payload, and the mock dies on its first `sendto` while the run still exits 0).
+
+The three lines and what each must read are in `probe-readings.md`.