Unify frontmatter under one AST node.

[samuel-williams-shopify]

Represent every supported frontmatter form with Node::Frontmatter. Untagged --- and +++ retain an empty authored info string and resolve to YAML and TOML through language(); explicit language hints use the same interface.

The parser, AST helpers, and renderers now handle one frontmatter type. All forms retain their raw body, including its trailing line ending, and use one Markdown serializer. When edited content requires replacing a dash or plus delimiter with a code fence, the serializer writes the implied language explicitly.

This changes the public AST: Node::Yaml, Node::Toml, and their structs are removed, and serialized ASTs use type: "frontmatter" with info and fence fields. Release notes describe the changes callers need to make.