An explicit key that is only a node property (`? &e` then `: &a`) parsed to an
`expectedNode` error. Unlike the implicit form (`&a : v`), the scanner emits no
synthesized empty scalar for an explicit key, so the block parser read the key
via parseNode, consumed the property, then hit the `:` value indicator with no
node and threw.
parseNode now treats a `:` in node position as an empty key carrying any
properties, reusing emitEmptyNodeOrThrow (the node it already produced for a
property at end of input) without consuming the `:`, so the mapping machinery
reads the `:` as the value indicator. To stay within the cyclomatic-complexity
limit, the scalar case is extracted to parseScalarNode.
yaml-test-suite PW8X now passes (parse-level 314/333 to 315/333). should-error
rejections and value-level matches are unchanged, and a value-level test pins
the produced tree for the anchored and tagged property-only explicit keys.
The block mapping parser required a ':' after every explicit key ('? key'),
so a key with no value, including several '? key' entries in a row (a set),
failed to parse with 'expected mapping value'. An explicit key with no
following ':' now takes an implicit null value, shared with the existing
'key:'-with-nothing-after path via one emitNullMappingValue helper.
Explicit keys with values and complex keys in general are unaffected. A
test that asserted '? [a, b]' followed by a block entry is a missing-value
error was corrected: the key is a valid null entry, and the trailing
sequence is now rejected as extra content (a second root node), which is
the more accurate diagnosis.
Measured against the official YAML test-suite, parse-level conformance
rises from 229/333 (68.8%) to 235/333 (70.6%), with several of the newly
parsed documents also value-correct and no regression.
