Block style permits only one mapping pair per line, so a scalar followed
by ':' after a block-level value on the same line (a: b: c, a: 'b': c,
x: { y: z }in: valid, or trailing content after a quoted value) is not a
new key but a forbidden inline nested mapping. The scanner synthesized an
implicit key for any scalar followed by ':', regardless of position, so
these parsed as nested mappings instead of failing.
The scanner now records when a line already holds a block-level mapping
pair and suppresses a further implicit key on that line, in plain and
quoted scalars alike. The flag is set only for an implicit-key ':' (one
preceded by its key on the same line): a ':' that opens its line is an
explicit- or null-key value indicator, whose value may be a compact
mapping (? k then : a: 1), so that, flow mappings, next-line nesting, and
colons inside scalars stay valid.
Measured against the official YAML test-suite, parse-level conformance
rises from 256/333 (76.9%) to 260/333 (78.1%) with no regression.
