ui: draw the sidebar tabs without the placeholder icon

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

docs/claude/probe-readings.md

@@ -479,9 +479,15 @@ 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=wide pills=4 labels=4 pages=4 size=308x34` (in the exit dump, beside `panels:`) — the
+- `switcher: policy=wide pills=4 labels=4 images=0 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
+  with text*, so `pills=labels=pages` says every page has a visible title. **`images` must be `0`**: it
+  counts *mapped* `GtkImage`s inside the switcher. `AdwViewSwitcherButton` builds an icon in every tab, and
+  a page with a title and no `icon-name` gets `image-missing` -- the `[x]` placeholder; the tabs hide it
+  (`keep_tabs_icon_free()`, which also covers pages `build_source_panels()` adds). A nonzero `images`
+  names a tab still drawing an icon or its placeholder (`images=pages` is the unfixed state), and
+  `pills`/`labels` never saw it. Read it on a source with extra pages (`--source probe` under
+  `UDPSCOPE_PROBE_SOURCE_KINDS`) as well as on UDPS. `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.