docs: horizontal plot split

docs/claude/verification.md

@@ -26,6 +26,15 @@ shortens `plot=` by the 36 px bar; and the `tour started: steps=` count. A self-
 `c572ec2` is no longer the baseline for those lines. The rest is the mock's own jitter (the sign of a
 `square y=[...]` or a `trig engine:` figure, the position of one `trigger: edge` line), which the
 normalisation does not remove -- anything else is a finding.
+For the horizontal plot split the same diff is taken against the commit before it (`18b91c8`, with
+`deps/plotter` at its pre-split pin `f79a97b`), and it is "unchanged except" exactly two things on the
+layout lines: **`buttons=`** grows (the editor now carries the cell `+` buttons and the column arrows:
+`3P + 2F + 2` for `P` plots in `F` figure rows, so `5 -> 7` for one row and `8 -> 12` for two), and the
+layout probe line and the exit dump's layout line gain the **`cells=[..] colweights=[..]`** fields.
+Everything else -- cursors, measurements, readouts, signal pages, math, trigger, help figures, the
+E-1608 -- reads as before; a difference there is a finding, not the feature. Take a second baseline run
+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
@@ -196,6 +205,19 @@ 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, 19 steps 250 ms apart (so a run needs
+  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=7500` 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
+  weights a row, fills the caps, round-trips a session through `collect_layout()` / `save_layout()` /
+  `load_layout()` / `apply_layout()` (and a copy with its `row`/`width` lines stripped), clicks the layout
+  editor's real cell `+` button, drives the column drag handler, and checks the signals page, the drop's
+  `row_at()` and the cursors pane's ruler labels. Run it once, serially, on its own mock port, from a
+  fresh `XDG_CONFIG_HOME` (the probe deletes the temp files `udpscope-split-probe.toml` and `.legacy` it
+  writes under `g_get_tmp_dir()`, but the editor's clicks schedule a real session save). Read it against
+  the `[split-probe]` section of [probe-readings.md](probe-readings.md); the run must still end with
+  `gtk warnings: 0`.
 - `[levels-probe] N measurement(s) over K channel(s), shares='...' -> W window(s), L level
   walk(s)` (stderr, `UDPSCOPE_PROBE_LEVELS_MS`) — **prices a measurement pane**, which is what a
   report of "it is slow with many measurements" has to become before it is a number. It adds one