docs: horizontal plot split

docs/claude/architecture.md

@@ -259,10 +259,12 @@ Layers to keep straight:
   `ChannelBuffer` is a `plot::data::Series` over a (value, time) ring pair.
 - `SlotSeries` (`src/channel.hpp`) — a trace's stable series object, rebound each frame
   to either a `ChannelBuffer` or an `EventSeries`.
-- `RowSet` (`src/plot_host.hpp`) — **one `plot::Figure`** of up to `kMaxRows` rows (added
-  and removed one at a time by the layout editor), each a
-  borrowed `plot::Plot*` with `kSlotsPerRow` fixed trace slots. The figure owns
-  everything the rows have to agree on: the x window (one x link group every row joins),
+- `RowSet` (`src/plot_host.hpp`) — **one `plot::Figure`** of up to `kMaxRows` **plots** (rows are added
+  and removed one at a time by the layout editor, and a row can be split into up to
+  `kMaxCellsPerRow` cells side by side), each plot a
+  borrowed `plot::Plot*` with `kSlotsPerRow` fixed trace slots; `row(i)` indexes the plots row-major,
+  `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.