ui: the trigger's wave is a full period again

The bold pass had turned the trigger into half a period — a signal rising above its level — and that
lost the thing the glyph is for: the crossings. A crest and a trough read as a wave; a hump reads as a
hill, and a trigger that shows no crossing names the "above the level" half of an event. The full-period
sine with the dashed level through it is restored, exactly as it was, and the Signals pill carries the
same wave again so the pill and the toolbar button stay one picture.

The Measure glyph keeps the bold redesign, which is the part that was better: two cursors (a cap over two
4px dashes) beside a ruler of two bands with three 2x4 marks and no end caps. The Signals pill also keeps
its two bold channel rows.

Verified: `toolbar icons: trigger=udpscope-trigger-symbolic(ok)`;
`page icons: measure=udpscope-tab-measure-symbolic(ok) signals=udpscope-tab-signals-symbolic(ok)`;
`switcher: policy=narrow pills=2 labels=0 names=2 icon=32x32 size=308x36`; `[side-tab-probe] … advanced=4`;
the Preferences dialog opened in the same run with no "not present"; `make test-ui` 14 files; zero
`Gtk-CRITICAL`. The icons' blob is 12352 bytes, still over the C99 limit, so the clean-build warning count
stays four.

README.md

@@ -2394,7 +2394,7 @@ now — they are listed so they are not mistaken for anything else:
   section was short of room, and finding one meant scrolling past the others. The built
   sidebar is a **Measure** tab (the cursors readout and the measurements) and a **Signals**
   tab (the trigger and the channels), **each a pill with its own glyph and no text** — the
-  cursors and the ruler for Measure, a signal rising above its trigger level and the channel list
+  cursors and the ruler for Measure, a sine crossing its trigger level and the channel list
   for Signals (the same picture the toolbar's trigger button carries), drawn as a
   32x16 pair so both motifs are legible at a glance — with the words kept as each pill's tooltip and
   accessible name. The design spec had the switcher showing *titles* and neither page an