A plain scalar at the document root failed to parse when it spanned
multiple lines at column 1 (`a\nb\nc`): the continuation check required a
continuation to be indented more than the scalar, but a root scalar has
nothing enclosing it, so its enclosing-block indent is -1 and a
continuation may start at column 1. The base indentation now returns -1 at
the root, and the continuation stops at a document marker (--- / ...) so a
scalar never folds across a document boundary.
A stream test that asserted two bare plain lines (first / second) were
rejected as extra content was corrected: that input is a valid multi-line
scalar; genuine extra content (a flow node followed by a scalar) is still
rejected, which the test now uses.
Measured against the official YAML test-suite, parse-level conformance
rises from 221/333 (66.4%) to 227/333 (68.2%), with four more documents
also now value-correct and no regression across the existing suite.