Complete ICU-backed regex case folds

[fglock]

Summary

• use ICU's Changes_When_Casemapped property as the complete, bounded candidate set for full folds
• supplement simple fold classes that ICU knows but the current JDK Pattern Unicode tables do not
• include every sibling code point sharing a full fold, such as U+00DF and U+1E9E
• add system-Perl and Java-level regression coverage without distribution preferences or copied Unicode tables

Results versus merged PR #911

Test	PR #911	This PR	Change
re/reg_fold.t	6925/7111	7106/7111	+181 passing
re/fold_grind_u.t	66/24309	66/7297	17,012 fewer emitted failures
re/fold_grind_d.t	22/7591	22/7141	450 fewer emitted failures
re/fold_grind_a.t	26/2147	26/1697	450 fewer emitted failures
re/fold_grind_aa.t	75/22811	75/22811	unchanged
op/ref.t	421/481	421/481	unchanged

The grind tests suppress successful internal subtests but emit each failure, so lower denominators represent additional passing internal cases.

Validation

• make — passed, all unit shards (2m36s)
• system Perl: prove src/test/resources/unit/regex_icu_full_casefold.t — 6/6 passed
• six-file perl_test_runner.pl comparison — no fold timeouts or incomplete fold files