shared-y: final review fixes

docs/superpowers/specs/2026-11-03-split-shared-y-design.md

@@ -50,7 +50,7 @@ one x window and draw one x axis. A split row reads as one wide plot with divide
   `place_cursors`; `sync_cursors` reads the dragged one back; `reset_cursors` and the measure level
   marks use the row's range. The cursors pane lists the pair once per figure row.
 - `row_unit` joins the units over all cells of the row, "" if mixed.
-- `sync_compare_axes` writes y once per figure row.
+- `sync_compare_axes` is unchanged: it writes x and y once per `RowSet` row. The comparison panes are flat single-cell rows, so that is once per figure row and no y sharing is involved.
 - `rebuild_all` turns the sharing on for the new `Figure` before the cells are populated.
 - Readouts and measurements stay per trace and per plot index; only the axis is shared.
 - Session file: unchanged (y ranges were never persisted).