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.

docs/claude/invariants-gtk-and-chrome.md

@@ -65,10 +65,14 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `udpscope-tab-measure-symbolic` is two cursors beside the ruler they read against — each a cap over two
   bold dashes, beside a ruler of two bands with three marks between them and **no end caps** (a closed
   frame reads as a rectangle, and costs a fifth of the ink on two lines that say nothing) — and
-  `udpscope-tab-signals-symbolic` is a signal rising above its level, beside two bold channel rows. They
+  `udpscope-tab-signals-symbolic` is a sine crossing its dashed level, beside two bold channel rows. They
   are not reuses, because a glyph the toolbar already carries means two things the moment a tab shows it
   too; instead the *trigger's* own icon (`udpscope-trigger-symbolic`) was redrawn to the Signals pill's
   picture, so the pill and the button that turns the trigger on are recognisably about the same thing.
+  The wave is a **full period and not a hump**: a crest and a trough are what make it read as a wave
+  rather than as a hill, and the level's dashes survive between the crossings. (A hump was tried and
+  reverted — it named the "above the level" half of a trigger and lost the crossings, which is the half
+  that matters.)
   Both are **32x16** composites drawn **bold** — one thick band, two thick dashes, few features — because
   at this size a thin wave or a 1px tick reads as *texture* rather than as a shape, which is what "not
   clear" looks like. Wider than tall on purpose: a 16px square cell carries one motif, not a pair. And