PR layer map

Produce a Mermaid diagram of which architectural layers a FormSG change touches, sized and styled to live in a GitHub PR description.

The output is a fenced ```mermaid block and nothing else. GitHub renders Mermaid natively in PR descriptions and comments, so the diagram travels with the PR rather than sitting behind a link that reviewers won't open. Keep it that way: no file tables, no test breakdowns, no companion artifact unless asked. A reviewer wants one glance that answers "how far does this reach?"

Generate it

# A PR, by number or URL
python3 scripts/classify_changes.py --pr 9893 --mermaid