Diff

flowfig diff <old> <new> reads the spec in two SVGs and lists each change. --md prints a Markdown list, which the Action puts in its comment. --json prints the changes for scripts.

$ npx flowfig diff first.svg first2.svg
box changed: api (label "API" -> "API gateway")
edge changed: api->db (label "read the user" -> "select the user")

The SVGs are first.svg from Your first figure and a copy with a new box label and a new edge label.