Routes the value and stream APIs through the ported scanner+parser (the
event API already was), so PureYAML now parses entirely on the libyaml
port. Adds the features needed to match and exceed the previous engine:
JSON UTF-16 surrogate-pair \u escapes, bare documents after an explicit
`...`, reserved-directive handling that still requires a following `---`,
and a 64-level nesting bound enforced on the event stream so deeply nested
input fails with nestingTooDeep instead of overflowing a recursive
consumer. Parser errors are translated to the diagnostic-aware ParseError,
recovering precise codes where identifiable and a readable malformed
fallback otherwise; error-reporting fixtures are rebased onto the port.
Canonical event conformance: 268 -> 271 of 308 (7Z25, HWV9, QT73).
393 tests pass.
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.
