- add build_wiki_nav deriving MkDocs nav from docs/wiki file structure
(index.md -> Home, numeric prefixes stripped and title-cased, nested
dirs become groups, natural ordering)
- add --wiki / --wiki-dir to build; wiki-only builds need no --module
- merge wiki nav before generated lib/api nav; wiki Home replaces the
nav spec Home entry
- add mkdocs.wiki.yml template fragment and nav/cli tests
- dogfood doc-forge's own docs/wiki and regenerate site output
