docs: split rows share one y axis

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

docs/claude/architecture.md

@@ -266,7 +266,8 @@ Layers to keep straight:
   `figure_row_count()` counts the rows. The figure owns
   everything the plots have to agree on: the x window (one x link group every plot joins),
   the live follow (the *group's* — a member's own axis must be `XFollow::Off`, since
-  `Figure::link_x()` refuses a `Live` member), the left gutter and the crosshair.
+  `Figure::link_x()` refuses a `Live` member), the y range of each figure row's cells (`set_share_row_y`: a split
+  row has one y axis, one ruler pair and one fit), the left gutter and the crosshair.
 
 - `ExportTable` (`src/export.hpp`) — the visible window as columns, collected from the
   snapshot or the live rings and handed to the `.npy`/`.csv` writers. Always the stored