ui: sidebar tab selector uses text labels instead of icons

The two drawn tab glyphs were judged not understandable enough; the
switcher goes to the wide policy with titles only, and the glyphs,
their gresource entries and the icon-naming code are removed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/claude/probe-readings.md

@@ -479,27 +479,14 @@ Two lines of the self-check output are assertions in disguise:
   substitutes a missing-image placeholder, the button still works, and every other line of the
   dump still passes. Changing an icon name is a one-line change in `build_toolbar()` and this
   is the only check that it names something that exists.
-- `switcher: policy=narrow pills=2 labels=0 names=2 icon=32x32 size=308x36` (in the exit dump, beside
-  `panels:`) — the two
-  sidebar tabs as the switcher actually draws them. **`pills=2 labels=0`** is the claim: two pills are on
-  screen and **no text is drawn** (`labels` counts *mapped labels with text*, because `narrow` keeps an
-  empty label in the pill's layout and a plain mapped count would read 2 either way). `names=2` is the
-  other half — both pills carry a tooltip and an accessible name set by `name_side_tabs()`, so an
-  icon-only pill is not an anonymous button — and `pills=0` means nothing is mapped, which makes the
-  rest of the line meaningless rather than a pass. `policy=narrow` is read rather than assumed. `icon=`
-  is the box the pills' `GtkImage` is *rendered* in, which proves the 32px override stuck: the tab
-  glyphs are 32x16 documents, and GDK paints an icon into a square box whatever the document's aspect,
-  so the artwork is drawn 32x16 at 1:1 inside that 32x32 box — letterboxed, not stretched, which is what
-  keeps its 2px bars crisp. It reads `0x0` on the first tick, where nothing has been allocated yet, and
-  is therefore read by the exit probe with the rest of the sidebar's geometry. `size=` is the bar's own
-  size, and it is the answer to "what did the wider glyphs cost": `36` is what it was with 16px icons,
-  because the pill's fixed height takes a 32px box without growing — the glyph gets twice the width for
-  no vertical room at all.
-- `page icons: measure=udpscope-tab-measure-symbolic(ok) signals=udpscope-tab-signals-symbolic(ok)` (in the exit
-  dump, beside `toolbar icons:`) — each tab's own glyph and `gtk_icon_theme_has_icon()`'s verdict, read
-  on the first tick for the same reason the toolbar's are. **Both must read `(ok)`**: the page titles are
-  empty on purpose, so an icon that does not resolve leaves a *blank* pill, and a blank pill looks
-  exactly like a page with nothing in it.
+- `switcher: policy=wide pills=4 labels=4 pages=4 size=308x34` (in the exit dump, beside `panels:`) — the
+  sidebar tabs as the switcher actually draws them, and they are **words**: `labels` counts *mapped labels
+  with text*, so `pills=labels=pages` says every page has a visible title. `pages` is read from the stack
+  (2 for UDPS, 3 for the probe source, 4 for the E-1608 with its Analog out and Digital I/O tabs). A
+  `labels` below `pills` is a page added with an empty title; `pills=0` means nothing is mapped, which
+  makes the rest of the line meaningless rather than a pass. `policy=wide` is read rather than assumed.
+  `size=` is the bar's own size: a text tab costs about 34 px of height, and a width of 308 shows the
+  four tabs fit the 320 px column.
 - `help layer: contextual=on hints=12 ids={events plot status cursors measures trigger trigger_advanced events_record trigger_search signals signals_config window} icon=dialog-information-symbolic(ok) hint=12x12; tour=idle steps=5 step=0 auto=1 requested=0` (in the exit
   dump, beside `switcher:`) — the help affordance layer as the last tick found it. **`hints=` must equal
   the number of ids**, and the ids are the surfaces the contextual help is on: eleven of the window