docs: horizontal plot split

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

@@ -52,7 +52,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   up front and rebinding via `SlotSeries`; a *name* change (legend text) is the one
   thing that forces a rebuild. Cursors are parked outside the axis range instead of
   removed.
-- **A `Figure`'s row count is fixed at construction**, so `RowSet::rebuild_all()` builds
+- **A `Figure`'s shape (its cells per row) is fixed at construction**, so `RowSet::rebuild_all()` builds
   a *new* figure and the area is handed it (`PlotArea::set_figure()`); it costs no GL
   state. That makes one path for "a row's traces changed" and "the row count changed",
   and it is why `rebuild_all()` restates a pinned event window (`pinned_pre_`/
@@ -105,6 +105,60 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   an equal division. `sync_layout_editor()` runs every frame on purpose:
   `gtk_widget_set_margin_*()` returns early on an unchanged value, so a settled layout costs
   the comparisons and nothing else, and a resize needs no notification.
+- **A figure row can be split into cells, and every plot index in the host is the flattened one.**
+  `RowSet::row_count()` is the number of **plots** and `row(i)`, `Row::plot`, the cursors, the
+  measurements, `bind()`, the readouts and the persistence layers all index plots row-major (row 0's cells
+  left to right, then row 1's); `figure_row_count()` is the number of figure rows, and
+  `cells_in_row()`/`first_plot_of_row()`/`figure_row_of()`/`cell_of()` translate. **Code that means "the
+  rows of the figure" -- the editor's row `+`, the row drag, the row weights, `fl.rows` -- must say
+  `figure_row_count()`**: with no split the two are equal and a slip passes every unsplit probe, which
+  is how a row weight once drifted off its row (see the `row-weights` reading). The caps are
+  `kMaxCellsPerRow` = 4 per row and `kMaxRows` = 8 **plots** in all (the constant bounds plots now, not
+  figure rows), so every per-plot structure sized for 8 is unchanged. `plot_label()` is what the UI
+  prints for a plot: the row's number (`2`) for a one-cell row, the number and a letter (`2a`..`2d`) for
+  a split one -- never a flat index, which is 0-based and says nothing about where the plot is.
+  What the cells share and what each keeps:
+  - **One time window.** Every cell of every row is a member of the one x link group, so a pan or zoom
+    in any cell moves them all, the crosshair is one x drawn in every cell (the hovered cell shows its
+    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.
+  - **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 *share it asks for*: a cell dragged to the limit realises
+    `64 + share * remainder` and reads wider (144 px in the split probe's `pre-insert0` line), so the
+    boundary does not follow the pointer one for one near either extreme.
+  - **A cell is removed with its traces, and a new cell is empty.** `insert_cell(fr, at)` pushes the
+    cells at and after `at` right *with their contents* (the same rule as `insert_row()`), and
+    `remove_cell(plot)` on a one-cell row removes the row. Both return false at a cap or when they would
+    leave no plot, and never grow past 4 / 8.
+  **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
+  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
+  `×`, 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
+  never by `set_row_key()`**: `row_key()` is a plot or boundary index, `layout_button()` finds a button by it,
+  and a cell `+` that also carried one would be found as a row button; `sync_layout_editor()` tells the kinds
+  apart by which key it finds. Like every editor control the
+  cell buttons are placed from `FigureLayout::cells`, divided by `device_scale()`, and the editor does
+  not draw while the figure's rects are not yet one per plot.
 - **The slide-over is the second thing placed over the plot, and it is one opaque widget, not a
   transparent area.** The same pick rule as the editor's buttons applies, harder: the panel is a
   `GtkRevealer` aligned to the end edge (`halign=END`, `valign=FILL`) at `kSlideOverWidth` = 360 px, so
@@ -189,6 +243,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   per-channel list, and why `format_elapsed()` steps to a coarser unit instead of
   letting the counter grow: each is a status field whose *width* would otherwise depend
   on the stream, and the status row is directly above the plot.
+  A split row keeps this rule: cell widths come from the weights and the 64 px floor and never from what
+  a cell's labels need, and the gutter stays one figure-wide maximum.
   The reservation goes on the **figure** (`Figure::set_min_y_gutter()`, added to the
   plotter for this), not on the rows: `Figure::layout()` clears every row's
   `set_min_y_gutter()` to measure what its own labels need, so a floor written onto the