layout editor: horizontal-resize icon, divider drag follows the pointer; x caption on every labelled plot; probes and docs

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

docs/claude/verification.md

@@ -36,8 +36,8 @@ E-1608 -- reads as before; a difference there is a finding, not the feature. Tak
 as a jitter control before believing one (a few mock readings, such as the sine's membership of the
 cursors mix list or a V/mV flip, move between two runs of the *same* build).
 A clean build emits **one** warning and no more: a `-Woverlength-strings` on the icons'
-generated GResource `.c`, whose blob is **9248** against C99's 4095-byte string-literal limit with
-the five action icons and the two app icons that remain (a note about a generated file, which GCC
+generated GResource `.c`, whose blob is **10146** against C99's 4095-byte string-literal limit with
+the six action icons and the two app icons that remain (a note about a generated file, which GCC
 itself supports). History: the help pages' generated `.c` (**516044** bytes) went when the manual
 became a Mallard set installed for Yelp; an unused `output_built` in `math_probe()` was deleted as
 dead code; and the last two, `-Wmissing-field-initializers` on `MathNode::source` in `math_probe()`,
@@ -205,8 +205,8 @@ precedent, not an exception.
   with, so a synchronous sequence would read every position one action stale. It clicks by
   emitting `::clicked`, never `gtk_widget_activate()`, which holds a `GtkButton` for its
   250 ms `ACTIVATE_TIMEOUT` and then emits into a button the rebuild has already destroyed.
-- `UDPSCOPE_PROBE_SPLIT_MS` — drive the horizontal plot split, 23 steps 250 ms apart (so a run needs
-  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=9500` at least; it enables and assigns the
+- `UDPSCOPE_PROBE_SPLIT_MS` — drive the horizontal plot split, 28 steps 250 ms apart (so a run needs
+  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=10500` at least; it enables and assigns the
   signals itself, so `UDPSCOPE_ENABLE_ALL_MS` is not needed). Each step restructures the cells,
   re-points the area at the new figure and lets the *next* step read a frame that was actually laid out
   with them, which is why every report line describes the previous step's change. It splits and