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.

README.md

@@ -2394,8 +2394,8 @@ 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 sine crossing its trigger level and the channel list for
-  Signals (the same picture the toolbar's trigger button carries), drawn as a
+  cursors and the ruler for Measure, a signal rising above 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
   `icon-name`; `narrow` alone does not do it (libadwaita still draws a label beside the icon,