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.

docs/claude/verification.md

@@ -16,9 +16,9 @@ xvfb-run -a env UDPSCOPE_EXIT_AFTER_MS=8000 UDPSCOPE_ENABLE_ALL_MS=1000 \
 Two baselines every run is read against. A clean build emits **four** warnings and no more: two
 `-Wmissing-field-initializers` on `MathNode::source` and `-Woverlength-strings` on **both**
 generated GResource `.c` files — the help pages' (**514660** bytes) and the icons', whose blob is
-**9842** against C99's 4095-byte string-literal limit with the six action icons and the two app icons
+**11309** against C99's 4095-byte string-literal limit with the seven action icons and the two app icons
 that remain (a note about a generated file, which GCC itself supports) -- re-measured when
-the tab work changed the set (that icon left, `udpscope-signals-symbolic` arrived, and this file's
+the tab work changed the set (that icon left, the two tab glyphs arrived, and this file's
 help page was rewritten), which did not move the count; re-measure by touching each `*.gresource.xml`
 under `src/` and rebuilding. The fifth warning that used to appear here was an unused
 `output_built` in `math_probe()`; it was **deleted** as dead code — redundant with the