A block key that is only a node property (`&a : v`, `!!str : v`) parsed to an
`expectedNode` error. The scanner synthesized an empty mapping key for a bare
`: v` line but not when the line's sole content before the `:` was an anchor or
tag, so no key marker was emitted and the parser rejected the `:`.
Track non-property content per line (a leading `&`/`!` does not count, and a
block entry opens a fresh node position) and synthesize the empty key whenever
only properties precede the `:`. The property tokens already bind to the
synthesized empty scalar through the existing mapping machinery, so no parser
change is needed. Relocate the synthesis helper to Scanner.Helpers to keep
Scanner under the file-length limit.
yaml-test-suite FH7J now passes (parse-level 313/333 to 314/333); PW8X still
needs the empty-anchored explicit-value form. should-error rejections and
value-level matches are unchanged, and a value-level regression test pins the
produced tree for each property-only-key shape.
A ':' that opens a line with no key before it is a mapping entry with a
null key (': value', a bare ':', or a null-keyed entry among others). The
scanner emitted the ':' with no preceding key, so the parser rejected it
as a missing node.
The scanner now synthesizes the empty key and its node for such a ':'. To
avoid mistaking an explicit key's value indicator for a null key, it tracks
a pending block-level '?' key: a ':' opening a line is that key's value
when one is pending, and only a synthesized null key otherwise. The flag
is set and cleared only at flow depth zero, so a ':' inside a flow complex
key ('? {a: b}' then ': c') does not disturb the pending block key.
Measured against the official YAML test-suite, parse-level conformance
rises from 283/333 (85.0%) to 287/333 (86.2%) with no regression.
