docs: split rows share one y axis

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

docs/claude/build-and-run.md

@@ -281,6 +281,28 @@ Cell widths use the height scheme: a floor of **64 px logical** per cell, the we
 the last cell closes the row exactly, and a figure too narrow for the floors shrinks the cells
 proportionally and drops the column gap. Only the bottom row's cells carry x labels and the x title
 (every other plot gets `XStrip::None`), and the left y gutter stays one maximum over the whole figure.
+
+The cells of a figure row can **share one y range**, symmetric to the x link
+(`docs/superpowers/specs/2026-11-03-split-shared-y-design.md`):
+
+- `Figure::set_share_row_y(bool)`, default off so every existing figure and `tests/test_figure.cpp`
+  read as before; the host turns it on in `RowSet::rebuild_all()` before the cells are populated. When
+  on, the cells of each figure row form an implicit y link group, rebuilt with the cells.
+- Link semantics: `layout()` runs a `sync_y_links()` that adopts a member's changed `y_range` as the
+  group's and pushes it to the others, so a wheel, pan, box zoom or axis drag (all `set_y_range`) needs
+  no controller change; a manual `set_y_range` on a member clears the group's auto flag. A member's own
+  autoscale is suppressed and the group owns the flag: it fits to the union of its members' visible
+  traces (cells with no trace are ignored; a log axis takes the union in axis space), and `fit_all` and
+  a double-click fit the group, not one cell. The group only follows on a laid-out frame, so a caller
+  that sets a range or a fit and reads it back needs one `layout()` in between.
+- `YStrip {Both, None}` and `Plot::set_y_strip_mode`, mirroring `XStrip`: `None` forces the y label
+  width, title width and reserved floor to 0. `Figure::layout` sets `None` on every cell but the first
+  of its row, structurally from the column index and never from content, and skips `None` cells in the
+  common-gutter maximum. A y axis drag can therefore start only in the leftmost cell.
+- **Cell placement is by plot width**: the row's y gutter comes off the row width first, then the cells
+  share the rest by the existing `floor + extra * w / total` rule, the floors applying to plot widths.
+  Equal weights give equal plot areas; the first cell's rect is gutter plus its plot width and the rects
+  still tile the row exactly.
 In the GTK4 and GLFW adapters the pointer is routed by **cell rect** (`row_in(rows, x, y)`: the
 containing cell, else the nearest by squared distance), one `input::Controller` per plot. The tests are
 the `figure_cell*` cases in `tests/test_figure.cpp` and `glfw_host_routes_by_cell`.