ui: bolder tab glyphs, and the trigger shares the Signals pill's picture

The previous pair were detailed miniatures — 1px ticks, 2px dashes, a 13-column wave — and at 32x16 that
reads as *texture* rather than as a shape, which is what "not clear" looks like. All three glyphs are
redrawn bold: one thick band, two thick dashes, few features.

- **Measure** is two cursors beside the ruler they read against. Each cursor is a cap over two bold
  dashes (4px), where three 2px dashes read as a dotted column; the ruler is two bands with three 2x4
  marks between them and **no end caps**, because a closed frame reads as a rectangle and costs a fifth
  of the ink on two lines that say nothing. This is the layout whose first attempt looked like two icons
  in a box — the difference is entirely the weight and the open ends.
- **Signals** is a signal rising above its level beside two bold channel rows. Half a period instead of
  a full one: at this size a full period is a zigzag and a hump is unmistakably a wave, and the level's
  marks survive on both sides of it instead of being swallowed by the crest.
- **The trigger's own icon** is now the same picture as the Signals pill, so the pill and the button that
  turns the trigger on are recognisably about the same thing.

Each was rendered and read back as an ink map at 1:1 and at 2x before it was installed, and three
candidates were compared for the Measure glyph (markers *on* a scale, cursors *over* a ruler, and this):
the first two turned the marks and the shafts into blobs at the crossings. Ink is 18%, 30% and 29%,
inside the range the icons above them set, and everything stays inset two pixels.

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 11774 bytes, still over the C99 limit, so the clean-build warning
count stays four.

src/icons/scalable/actions/udpscope-trigger-symbolic.svg

@@ -1,26 +1,22 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!-- The trigger: a sine and the dashed level it is looked for at, which is what a trigger *is* on a
-     waveform that is not a step. It used to be a rising edge, and one edge names only one of the four
-     shape triggers the panel offers — a wave crossing a level names the trigger's job instead. One
-     period, one-pixel stairs, the way the plot rasterises its own traces, with the level through the
-     centre so the crest and the trough stand clear and the crossings fall on the steep parts. Inset
-     two pixels, like the tabs' glyphs, so no art is flush with an edge a button might clip. -->
+<!-- The trigger: a signal rising above the level it is looked for at, with the level showing on both
+     sides of it. One event, which is what the trigger is for. Bold on purpose: at this size a
+     thinner wave reads as texture, and the level has to survive as two clear marks rather than as a
+     dotted line. Inset two pixels like the tabs' glyphs, so no art sits flush with an edge a button
+     might clip. Whole-pixel stairs, the way the plot rasterises its own traces. -->
 <svg height="16px" width="16px" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
-  <path fill="#2e3436" d="M2 7 H3 V10 H2 Z
-                          M3 5 H4 V8 H3 Z
-                          M4 3 H5 V6 H4 Z
-                          M5 3 H6 V6 H5 Z
-                          M6 4 H7 V7 H6 Z
-                          M7 6 H8 V9 H7 Z
-                          M8 8 H9 V11 H8 Z
-                          M9 10 H10 V13 H9 Z
-                          M10 11 H11 V14 H10 Z
-                          M11 11 H12 V14 H11 Z
-                          M12 9 H13 V12 H12 Z
-                          M13 7 H14 V10 H13 Z
-                          M2 7 H5 V9 H2 Z
-                          M7 7 H10 V9 H7 Z
-                          M11 7 H14 V9 H11 Z"/>
+  <path fill="#2e3436" d="M3 7 H4 V11 H3 Z
+                          M4 5 H5 V9 H4 Z
+                          M5 4 H6 V8 H5 Z
+                          M6 3 H7 V7 H6 Z
+                          M7 2 H8 V6 H7 Z
+                          M8 2 H9 V6 H8 Z
+                          M9 3 H10 V7 H9 Z
+                          M10 4 H11 V8 H10 Z
+                          M11 5 H12 V9 H11 Z
+                          M12 7 H13 V11 H12 Z
+                          M2 8 H3 V10 H2 Z
+                          M13 8 H15 V10 H13 Z"/>
 </svg>