ui: the Measure pill is two cursors on a scale, not a boxed ruler beside them

The previous Measure glyph was two icons sharing a box: a heavy closed ruler frame on the right and
two light caps on the left, with nothing relating them. It is now one instrument — two cursors
*standing on* the scale they read against, each marker centred on one of the scale's marks, so the
pair reads the way the pane works: bracket a window, read it against the axis.

Three things the render said, each fixed:

- **The closed frame was most of the ink.** Two lines and two end caps cost about a fifth of the
  glyph's ink and say nothing a ruler's marks do not, so the scale is now one band with alternating
  marks — long, short, long — hanging above it.
- **The caps were dashes, not grips.** 5x2 at 32px read as a horizontal stroke; they are 5x3, like the
  caps on the toolbar's cursors icon.
- **The ticks were finer than everything else.** At 1px they were thinner than the 2px weight every
  icon here uses, so they are 2px, and every mark alternates in height the way a ruler's do.

The Markers' shafts are dashed and meet the band rather than hovering over it, and the whole glyph is
inset two pixels like the others, so nothing is flush with an edge that a button's padding or a crop
takes a pixel off. Ink: 27%, inside the range the icons above it set.

Verified: `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" (the plain ruler it uses is untouched);
`make test-ui` 14 files; zero `Gtk-CRITICAL`. The icons' blob is 12624 bytes, still over the C99 limit, so
the clean-build warning count stays four.

docs/claude/invariants-gtk-and-chrome.md

@@ -65,8 +65,10 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `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 **32x16** composites, one motif per half: the Measure pill is the two cursors beside the ruler, the
-  Signals pill a sine with a dashed level through it beside the channel list. That picture is shared on
+  are **32x16** composites: the Measure pill is two cursors *standing on* the scale they read against — the
+  one band with alternating marks, each marker centred on one of them, which is one instrument rather
+  than two icons in a box — and the Signals pill a sine with a dashed level through it beside the
+  channel list. That picture is shared on
   purpose: the trigger's *own* toolbar icon (`udpscope-trigger-symbolic`) is the same sine and the same
   level, one period of one-pixel stairs, so the pill and the button that turns the trigger on are
   recognisably about the same thing. The trigger glyph used to be a rising edge, and one edge names only