A directive the processor does not recognize (e.g. %FOO) before a
document is now ignored, matching the YAML spec (emit a warning, otherwise
no effect), rather than failing the whole document with an
unsupportedDirective error. %YAML and %TAG handling is unchanged, and a
directive after content, where a directive cannot legally appear, is still
rejected via the existing guard.
Two tests and a fixture that pinned the old reject-everything behavior are
updated: the unknown-directive case moves from the unsupported-gaps list,
and the scanner-error test now exercises a directive-after-content case
(still an error) rather than an unknown directive before content.
Measured against the official YAML test-suite, parse-level conformance
rises from 227/333 (68.2%) to 229/333 (68.8%), with the two newly-parsed
documents also value-correct and no regression.