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-tab-signals-symbolic.svg

@@ -1,32 +1,27 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!-- The Signals tab: the toolbar's trigger picture — a sine with a dashed level through it — beside
-     the channel list. Sharing that picture is the point: the pill and the toolbar button that turns
-     the trigger on should be recognisably about the same thing. Three channel rows on the right, on
-     the left half's centre line, and everything inset two pixels, because art flush with the cell
-     edge is what a button or a crop clips. -->
+<!-- The Signals tab: the toolbar's trigger picture — a signal above its level — beside the channel
+     list. Sharing that picture is the point: the pill and the button that turns the trigger on should
+     be recognisably about the same thing. Two channel rows on the right, drawn bold for the same
+     reason the wave is, and everything inset two pixels. -->
 <svg height="16px" width="32px" viewBox="0 0 32 16" xmlns="http://www.w3.org/2000/svg">
-  <path fill="#2e3436" d="M2 6 H3 V10 H2 Z
-                          M3 4 H4 V8 H3 Z
-                          M4 3 H5 V7 H4 Z
-                          M5 2 H6 V6 H5 Z
-                          M6 3 H7 V7 H6 Z
-                          M7 4 H8 V8 H7 Z
-                          M8 6 H9 V10 H8 Z
-                          M9 8 H10 V12 H9 Z
-                          M10 9 H11 V13 H10 Z
-                          M11 10 H12 V14 H11 Z
-                          M12 9 H13 V13 H12 Z
-                          M13 8 H14 V12 H13 Z
-                          M14 6 H15 V10 H14 Z
-                          M2 7 H5 V9 H2 Z
-                          M7 7 H10 V9 H7 Z
-                          M12 7 H15 V9 H12 Z
-                          M18 2 H21 V5 H18 Z
-                          M23 3 H30 V5 H23 Z
-                          M18 7 H21 V10 H18 Z
-                          M23 8 H30 V10 H23 Z
-                          M18 12 H21 V15 H18 Z
-                          M23 13 H30 V15 H23 Z"/>
+  <path fill="#2e3436" d="M3 7 H4 V12 H3 Z
+                          M4 6 H5 V11 H4 Z
+                          M5 4 H6 V9 H5 Z
+                          M6 3 H7 V8 H6 Z
+                          M7 2 H8 V7 H7 Z
+                          M8 2 H9 V7 H8 Z
+                          M9 2 H10 V7 H9 Z
+                          M10 2 H11 V7 H10 Z
+                          M11 3 H12 V8 H11 Z
+                          M12 4 H13 V9 H12 Z
+                          M13 6 H14 V11 H13 Z
+                          M14 7 H15 V12 H14 Z
+                          M2 8 H3 V10 H2 Z
+                          M15 8 H17 V10 H15 Z
+                          M18 3 H23 V8 H18 Z
+                          M24 4 H30 V7 H24 Z
+                          M18 10 H23 V15 H18 Z
+                          M24 11 H30 V14 H24 Z"/>
 </svg>