ui: each tab's glyph is a pair, not one icon

The Measure tab is now `udpscope-tab-measure-symbolic` — the two cursors standing across a ruler — and
the Signals tab is `udpscope-tab-signals-symbolic` — the trigger's rising edge over the channel list it
acts on. Each is a 16px composite rather than one of the icons the toolbar carries: a glyph the toolbar
already uses for one thing means two things the moment a tab shows it too, which is why the ruler alone
and the trigger alone were both wrong here.

The plain ruler stays exactly where it was. `udpscope-measure-symbolic` is the Preferences window's Plot
page icon and is untouched; the tab's pair is a new file beside it, so nothing that already meant "the
measurement ruler" changed meaning. Seven action icons bundled, nine with the two app icons.

Both composites are at the limit of what 16px holds — four features each — so each was drawn and then
*measured*: rasterized and read back as an ink map, since the shapes have to be checked as pixels and the
house range is 25-36% ink (trigger 25, cursors 27, ruler 36). The Signals list is two rows and not three,
so the edge and the list get half the cell each; its bars are all one length, because unequal ones read as
a bar chart and a chart is not a list. The Measure ruler's ends are open on purpose: a closed frame puts
four more pixels of ink at the left and right edges, which is what turns a ruler into a rectangle at this
size. First drafts came out at 39% and 44% ink and were rebalanced to 32% and 36%.

Verified: `page icons: measure=udpscope-tab-measure-symbolic(ok) signals=udpscope-tab-signals-symbolic(ok)`;
`switcher: policy=narrow pills=2 labels=0 names=2`; `[side-tab-probe] … advanced=4` (switching by page
name still works); the Preferences dialog opened in the same run with no "not present"; `make test-ui` 14
files; zero `Gtk-CRITICAL`. The icons' GResource blob is now 11309 bytes, still over C99's 4095-byte
string-literal limit, so the clean-build warning count is still four.

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

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!-- The Signals tab: the trigger *and* the channel list it acts on, in one glyph. A rising edge
+     crossing a dashed level is the trigger as the toolbar already draws it; below it, two rows of a
+     colour chip and a name are the channel list, which is what the page mostly is. Two rows and not
+     three, so the edge and the list each get half the cell: at 16px a third row costs the edge its
+     clearance and the whole thing turns into a grid. Whole pixels, filled `path`s wound the same
+     way, and the 2px bars match the house stroke weight. -->
+<svg height="16px" width="16px" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
+  <path fill="#2e3436" d="M6 1 H8 V6 H6 Z
+                          M6 1 H15 V3 H6 Z
+                          M0 5 H2 V7 H0 Z
+                          M3 5 H5 V7 H3 Z
+                          M12 5 H14 V7 H12 Z
+                          M1 9 H4 V12 H1 Z
+                          M6 10 H14 V12 H6 Z
+                          M1 13 H4 V15 H1 Z
+                          M6 13 H14 V15 H6 Z"/>
+</svg>