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/invariants-gtk-and-chrome.md

@@ -55,14 +55,21 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   binary.** No icon theme has a glyph for an oscilloscope trigger, a pair of cursors or a
   measurement ruler, and the stock names those buttons used to carry said something else
   entirely: a *record dot* for the trigger and three different **magnifiers** for cursors,
-  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,events,resize-rows,signals}-symbolic.svg`
-  are the six action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
+  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,events,resize-rows,tab-measure,tab-signals}-symbolic.svg`
+  are the seven action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
   user; `udpscope-measure-symbolic` **stayed**, because the Preferences window's Plot page carries it --
   an icon with a live user is not an orphan, whatever the toggle that introduced it did -- and it is
-  *also* the Measure tab's icon. `udpscope-signals-symbolic` was drawn for the Signals tab: three rows of
-  a colour chip and a name, which is what that tab mostly is, and deliberately *not* a second copy of the
-  trigger glyph (one shape with two meanings in one window is worse than a new one). Its bars are all one
-  length because unequal ones read as a bar chart, and a chart is not a list. Beside them in
+  the Preferences window's icon, not the tab's.
+
+  The two **tabs** got glyphs of their own, and each is a *pair* rather than one of the icons above:
+  `udpscope-tab-measure-symbolic` is the cursors and the ruler they read against, and
+  `udpscope-tab-signals-symbolic` is the trigger's edge above the channel list it acts on. They are not
+  reuses because a glyph the toolbar already carries means two things the moment a tab shows it too. Both
+  are 16px composites with four features each, which is about the limit of this size: the Signals list is
+  two rows and not three, so the edge and the list get half the cell each, and the bars in it are all one
+  length because unequal ones read as a bar chart, and a chart is not a list. Their ink fractions (32% and
+  36%) sit inside the range the icons above set, which is the check that a composite has not turned into a
+  blot. Beside them in
   `src/icons/scalable/apps/` are the two
   **application** icons, `org.udpscope.app.svg` (128px colour artwork) and
   `org.udpscope.app-symbolic.svg` (16px). The `apps` context is where an icon theme keeps an
@@ -71,7 +78,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   into `$XDG_DATA_HOME/icons/hicolor/scalable/apps` and `.../symbolic/apps` — those installed
   copies are what makes `Icon=org.udpscope.app` in the desktop entry resolve (a desktop shell
   cannot read a GResource compiled into the binary).
-  All eight are bundled by `src/icons/udpscope.gresource.xml` (CMake runs
+  All nine are bundled by `src/icons/udpscope.gresource.xml` (CMake runs
   `glib-compile-resources`) and reached through **one**
   `gtk_icon_theme_add_resource_path()` in `run_ui()`, right after `adw_init()` — not at the top
   of `build_toolbar()`, where it used to be: there are two consumers now (the window's own icon,