v4.0.0

Changed

• MDD 3 directive syntax. A block directive is written :::letterhead … ::: and a self-closing one ::page-break / ::section-break: the remark-directive grammar, so any remark-directive pipeline (notdocs, docs sites) parses MDD without this package's paragraph scanner, and a document round-trips through such a pipeline byte for byte. The structure plugin maps containerDirective and leafDirective nodes straight to semantic containers when remark-directive runs before it, and still scans paragraphs when it does not.
• The MDD 2 spelling is read for one more major version. ::name … ::, ::page-break ::, ::: section-break ::: and their block forms parse as before; the validator reports each with a LEGACY_DIRECTIVE_SYNTAX warning that points at mdd-migrate. A canonical :::name block is not closed by a legacy :: and vice versa.
• Validation messages spell directives the MDD 3 way (:::letterhead, ::page-break).

Added

• LEGACY_DIRECTIVE_SYNTAX error code (a warning).
• The four example documents from @markdownkit/mdd in canonical syntax as test fixtures, checked to validate clean.