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-plot-and-layout.md

@@ -157,13 +157,13 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
     (`kColGap`, 6 px); the editor's drag is stricter (below). The floors and the weights act on **plot widths**: the row's y gutter (carried by the first
     cell) is taken off the row width first, then the cells share the rest, so equal weights give equal plot
     areas and the first cell's rect is its gutter plus its plot width. A weight is a share of what is
-    left *after* the gutter and the floors, so cells of `{3, 1}` read 546:207 (rects) in a 756 px
+    left *after* the gutter and the floors, so cells of `{3, 1}` read 524:229 (rects) in a 756 px
     figure, not 3:1, and the editor's 96 px drag floor is a floor on the plot *width*, not on the share. Because a cell's width is
     `64 + extra * weight / total` and not proportional to its weight, `layout_col_apply()` does not scale
     the pair's weights by the width ratio: it keeps the pair's weight sum `S` from the drag's *start*
     (`layout_col_drag_start`) and gives the left cell `S * (want_left - 64) / (pair - 128)`, clamped to
     `[0.001, 0.999]` (the plotter refuses non-positive weights; `0.5` if the pair is not wider than 128).
-    `want_left` and the pair are plot widths (`layout_col_drag_begin` subtracts the first cell's `y_axis_width + y_title_width` from its rect); `want_left` is the pointer's width clamped to `[96, pair - 96]` logical px. The divider therefore
+    `want_left` and the pair are plot widths (`layout_col_drag_begin` reads each plot's `layout().axes.w`, after a `layout()` on the current figure, so the gutter and the inter-axes gap are never counted as weight); `want_left` is the pointer's width clamped to `[96, pair - 96]` logical px. The divider therefore
     follows the pointer, the shrinking cell reads 96 px at the extreme and the pair keeps its total width
     (the split probe's `col-drag` line asserts all three). `kPlotterMinCellPx` (64) mirrors the plotter's
     `kMinCellWidthLogical`; both are logical px (the plotter scales it by `ui_scale`, which is
@@ -176,9 +176,12 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   **The editor's controls** (`rebuild_layout_editor()`): a cell `+` (`kLayoutCellKey`, tooltip "split
   here") at a row's left edge, at its right edge and at every internal boundary -- `n + 1` of them for a
   row of `n` cells, so a one-cell row can be split from either side -- inserting at that position; a drag
-  arrow (`kLayoutColKey`, `go-next-symbolic`) at each internal boundary that moves the boundary between
+  double-headed arrow (`kLayoutColKey`, `udpscope-resize-cols-symbolic`) at each internal boundary that moves the boundary between
   its two neighbours, **each floored at `kMinCellPx` = 96 px logical** (or half the pair when the pair is
-  narrower) and measured from the two widths at drag start, so the drag does not compound; and the per-plot
+  narrower) and measured from the two widths at drag start, so the drag does not compound. **The handle follows the divider it
+  drags, so the gesture's offset shrinks by the handle's own displacement**: `on_layout_col_update` adds it back
+  (`layout_col_total_offset(offset, handle_x_now, handle_x_at_begin)`, the handle's x in overlay coordinates, stored by
+  `on_layout_col_begin` in `layout_col_drag_handle_x0`) before multiplying by the scale; without it the divider settles at half the pointer's travel. The per-plot
   `×`, which removes the cell in a split row ("remove this plot") and the row in a one-cell row ("remove
   this row"), insensitive when it would leave no plot. Cell `+` is insensitive at 4 cells in its row or 8
   plots in all; the row `+` is insensitive at 8 plots. **The cell `+` is keyed by `kLayoutCellKey` (figure row * 8 + position + 1) and