Each pattern facet value is now compiled when the schema is, instead of only
lazily at instance time (try?) where an unparseable pattern was silently
ignored. A structurally malformed pattern (unbalanced parentheses, a quantifier
with nothing to repeat) is rejected.
The check is conservative against over-rejection: it rejects only the
unambiguous structural RegexError kinds (.unbalanced, .danglingQuantifier) and
tolerates the kinds that fire on otherwise-valid patterns the engine cannot
fully handle (.empty for the empty pattern, .unsupported for an untabulated
\p{Is...} block such as a surrogate or historic block, .badQuantifier for the
lenient {,m} form, and .badEscape/.badClass which could fire on valid character-
class subtraction). So a{2,1} and a trailing backslash remain accepted (an
under-rejection), but no valid schema is rejected.
XSTS: invalid schemas accepted 1129 to 1092 (-37); valid schemas rejected held
at 72; instance buckets unchanged.
