ui: the trigger glyph is a sine crossing its level, and every glyph is inset

The trigger's toolbar icon was a rising edge, which names only one of the four shape triggers the panel
offers. It is now a sine with a dashed level through it — the trigger's actual job, find the signal where
it crosses this — and the Signals tab's pill carries the *same* picture beside the channel list, so the
pill and the button that turns the trigger on read as the same idea. Both are one period of one-pixel
stairs, the way the plot rasterises its own traces.

Everything is inset two pixels now. The previous drafts ran their level and their bars flush to the cell
edge, which is what makes art look shaved wherever a button's padding or a crop takes a pixel: nothing in
either tab glyph, or in the trigger's, touches an edge.

Each shape was drawn and then read back as an ink map rather than trusted — the first sine came out as a
thin antialiased diagonal (a polygon through whole-pixel vertices has diagonal *edges*), which is why the
band is now per-column stairs; the level-through-the-centre arrangement was chosen over level-above-the-
centre after rendering both, since at these sizes a level nearer the crest merges with it into a blob. The
four glyphs measure 18%, 22%, 25% and 25% ink, inside the range the icons above them set.

Verified: `toolbar icons: trigger=udpscope-trigger-symbolic(ok)` and
`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`; `make test-ui` 14 files; zero `Gtk-CRITICAL` and no missing icon. The
icons' GResource blob is 12408 bytes — the SVG sources are embedded, comments included — still over the
C99 limit, so the clean-build warning count stays four.

src/icons/scalable/actions/udpscope-tab-signals-symbolic.svg

@@ -1,21 +1,28 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!-- The Signals tab, as two motifs side by side in a 32x16 cell: the trigger on the left (a rising
-     edge crossing a dashed level, the shape the toolbar's trigger button has) and three rows of a
-     colour chip and a name on the right, which is the channel list the tab mostly is. Wider than
-     tall because it has to hold *two* motifs: a 16px square cell can carry one, and squeezing the
-     pair into it is what turns them into a smear. The pill renders this at 32, i.e. 1:1, so the 2px
-     bars stay crisp; a fractional scale would blur them, which is why every coordinate is a whole
-     pixel. GDK paints an icon into a *square* box whatever the document's aspect, so this pair is
-     letterboxed in a 32x32 box rather than stretched (measured: a 32x16 render inside a 32 request).
-     All subpaths are filled `path`s wound the same way. -->
+<!-- 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. -->
 <svg height="16px" width="32px" viewBox="0 0 32 16" xmlns="http://www.w3.org/2000/svg">
-  <path fill="#2e3436" d="M4 2 H6 V12 H4 Z
-                          M6 2 H14 V4 H6 Z
-                          M0 7 H2 V9 H0 Z
-                          M8 7 H10 V9 H8 Z
-                          M12 7 H14 V9 H12 Z
+  <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