A directive following a document-end marker ('...') begins the next
document's directive section, but the scanner rejected it: the
isDocumentClosed flag set by '...' tripped the directive guard, so a
multi-document stream where each document carries its own directives
('%TAG ...' / '--- doc' / '...' / '%TAG ...' / '--- doc') failed.
A directive now reopens the stream when it follows '...', clearing the
closed flag, while a directive after document content within the same
document stays rejected via the content guard. The directive still
requires its own '---', so a directive after '...' with no document start
is still an error. Multi-document streams already composed through the
stream mode; this removes the scanner-side block on per-document
directives.
Measured against the official YAML test-suite, parse-level conformance
rises from 278/333 (83.5%) to 283/333 (85.0%) with no regression.