A `---` or `...` at column 0 ends an active block scalar rather than being
consumed as its content. The scanner now closes the scalar's indentation at the
marker, dedenting back to the document level, so the token stream stays balanced:
the dedent precedes the documentEnd. Previously the scalar's indent lingered past
the marker and surfaced as a stray late dedent plus a phantom empty document, and
a following directive was misread as a scalar.
Fixes suite case W4TN (parse-level conformance 315 -> 316); adds tests.
Mihaela M
8112954f4c