docs: split rows share one y axis

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

docs/claude/invariants-plot-and-layout.md

@@ -123,27 +123,40 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
     y), and the **vertical cursor pair is one shared time drawn in every cell** (`cursors_aligned`).
     A split is an arrangement of more plots, not a second time scale: nothing about the time axis changes
     (see [invariants-time-axis-and-window.md](invariants-time-axis-and-window.md)).
-  - **Per-cell y.** Each cell has its own y axis (auto or fixed), its own horizontal rulers and its own
-    fit; a wheel or axis drag acts on the cell under the pointer, an x gesture on any bottom-row cell
-    moves the shared window. The cursors pane's ruler label is `plot 2a` for a plot of a split row and
-    `row N` (the plot index) for one of an unsplit row, so the pane reads the same as before until a row
-    is split.
+  - **One y per figure row.** The cells of a figure row share one y range (the plotter's
+    `Figure::set_share_row_y(true)`, which `rebuild_all()` sets before the cells are populated): one y
+    axis (auto or fixed), one pair of horizontal rulers, one fit. A wheel or axis drag in any cell moves
+    the row, an x gesture on any bottom-row cell moves the shared window. The host therefore acts on the
+    **figure row** of a plot it is given (`set_y_auto`, `set_y_range`, `fit_all`, the Auto/Fixed dialog,
+    `row_unit`, which joins the units of every cell and reads "" if they differ). The rulers are stored on
+    the row's first plot index, copied to the other cells by `sync_cursors()` / `insert_cell()` /
+    `rebuild_all()` and placed in every cell by `place_cursors()`. The cursors pane lists **one ruler line
+    per figure row**: `row N` with N the 1-based figure row for a split row, and the 0-based plot index
+    for an unsplit row (the pane read that before splitting existed). Readouts and measurements stay per
+    trace and per plot index; only the axis is shared.
   - **Bottom-row x labels only.** The plotter gives every plot that is not in the last figure row
     `XStrip::None`; the cells of the bottom row each keep their strip, so a middle-drag on the axis
     works in any of them. The `t0` caption goes on those cells too.
-  - **One left gutter.** The y gutter is one maximum for the whole figure with the same reserved floor
-    as before (`Figure::set_min_y_gutter()`), **not a per-column one**: a per-column gutter would make a
-    cell's width depend on its labels, which "layout must not depend on content" forbids. A cell whose y
-    labels are narrower than the gutter simply has a wider margin.
-  - **Widths are weights with a floor.** The plotter keeps 64 px logical per cell and a column gap
-    (`kColGap`, 6 px); the editor's drag is stricter (below). A weight is a share of what is
-    left *after* the floors, so cells of `{3, 1}` read 533:220 in a 756 px figure, not 3:1, and the
-    editor's 96 px drag floor is a floor on the *width*, not on the share. Because a cell's width is
+  - **One left gutter, on the first cell only.** The y gutter is one maximum for the whole figure with the
+    same reserved floor as before (`Figure::set_min_y_gutter()`). It is carried by the first cell of each
+    row alone: `Figure::layout` gives every other cell `YStrip::None` from its column index (structurally,
+    never from what it holds), which forces its y label width, title width and reserved floor to 0. A
+    `None` cell contributes 0 to the common maximum, so a split row's non-first cells cannot widen the
+    gutter and a row's gutter does not depend on how many cells it has. A per-column gutter sized by the
+    labels would make a cell's width depend on its content, which "layout must not depend on content"
+    forbids. A row whose labels are narrower than the gutter simply has a wider margin. The weights apply
+    to **plot widths**, not cell rects (below).
+  - **Widths are weights with a floor.** The plotter keeps 64 px logical per plot and a column gap
+    (`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
+    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` 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` 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
     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