A ':' value indicator with no preceding key is also valid inside a flow
mapping, denoting an entry whose key is empty (null). libyaml hands the
bare VALUE token to its node parser and fails with 'did not find expected
node content'. The flow-mapping-key parser now emits the empty scalar key
and lets the value state consume the indicator.
Fixes '{ : }' and '{ key: value, : empty key }', completing the
multi-document NKF9 case. Port score 282 -> 283.
Ports parser.c: the one-event-at-a-time state machine that turns the
scanner's token stream into the YAML event stream. Covers stream and
document framing, directive processing with %TAG handle resolution and
default handles, the node property/tag/anchor production, block and
indentless sequences, block mappings, and flow sequences and mappings
including the single-pair `?` mapping forms.
Output verified against the libyaml reference event dumper: the empty
mapping value, %TAG resolution, and shorthand tags all match byte for
byte. 331 tests, no regression.
