docs: pick the natural axis for the 3-feeds-1 case (column for R/L edges, row for T/B)

`align row` for three databases that all connect `R --> L:mcp` linearises them into
a chain pointing into MCP rather than producing the intended fan-in. The natural
fit for "three things feeding one downstream node" depends on the port pair:

- column when the edges share a horizontal port pair (e.g. R --> L) — siblings
  stack vertically with parallel horizontal arrows reaching the downstream node.
- row when the edges share a vertical port pair (e.g. B --> T) — siblings sit
  in a horizontal row with parallel vertical arrows fanning down.

Updated the docs example and the Cypress test to match this guidance.

Verified visually: arch-verify-column-row.png shows DB1/DB2/DB3 in a clean
vertical stack with three parallel arrows into MCP, and Source 1/2/3 in a clean
horizontal row with three parallel arrows fanning down into Processor.