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/invariants-time-axis-and-window.md

@@ -73,8 +73,9 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   crawled across the plot under a pan and drifted under a zoom, which is what "[t0=...s] move
   slightly around when user zoom/pan" was. Three things follow. The **text is the host's**, because
   the caption has to name the anchor precisely enough that label + caption is the exact absolute
-  time and `format_time()` would round it to three figures (`"t0 = %.6g s"`); only the **bottom
-  row** of the figure carries one, every other row being set to `""`; and the renderer measures it
+  time and `format_time()` would round it to three figures (`"t0 = %.6g s"`); only a plot
+  whose x strip draws labels** (`plot::x_strip_has_labels(x_strip_mode())`: the bottom row, and any row above it whose
+  columns differ from the row below's) carries one, every other plot being set to `""`; and the renderer measures it
   rather than `Plot::layout()` — it reserves no space and nothing can be picked on it, so there is
   no rect to read back, and a caption explaining the x labels must not be able to move the layout
   those labels were measured into. The guards are the library's
@@ -86,7 +87,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   path a resume takes). Pinned — a held event, a filling capture or a paused view — it is the
   **pin's** full range, which is the snapshot's own stamps and may be legitimately *narrower*
   than the requested window. It must write the per-row **member** axes as well as the link group:
-  `hold_pinned_window()` prefers a member that differs from `pinned_stated_`, which is how it
+  `hold_pinned_window()` prefers a member that differs from `pinned_stated_` (it scans **every**
+  row's member, in order, since a labelled strip on a row other than the first takes x gestures too), which is how it
   tells a real gesture from its own echo, so writing the group alone is reverted to the zoomed
   range on the next frame. `fit_all()` is the y fit plus this, shared rather than duplicated so
   the two cannot drift apart.