plot host: split rows share y, rulers per row

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

docs/claude/verification.md

@@ -205,15 +205,15 @@ 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
+- `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
   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
+  `row_at()`, the cursors pane's ruler labels and the shared y axis of a split row (`shared-y`). 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