A block implicit key must lie on a single line, so a quoted scalar whose
opening and closing quotes sit on different lines cannot be one. The
scanner synthesized the implicit key regardless, so '"c\n d": 1' and the
single-quoted form parsed as a mapping with a folded key.
When synthesizing the implicit key for a quoted scalar followed by ':',
the scanner now rejects the construct if the scalar spanned a line break.
The restriction is block-only: a flow mapping key may span lines, so the
check is gated on block context, and a multiline quoted value, a single-
line quoted key, and an explicit key (which carries no ':' directly after
the scalar) are all unaffected.
Measured against the official YAML test-suite, parse-level conformance
rises from 261/333 (78.4%) to 264/333 (79.3%) with no regression.
