Markdown as Canonical Source

TileDown treats Markdown as the author-owned format. The parser reads front matter, CommonMark blocks, and typed tile directives into a structured model.

The formatter can then rewrite a document to one canonical profile:

swift run tiledown fmt --check source.md
swift run tiledown fmt --write source.md

That keeps the source stable enough for reviews and future visual editing while still being a plain text format.