Reject UTF-16 surrogate values in Character converter

[rootvector2]

the Character converter in putDefaultMap accepts \uD800 through \uDFFF because Character.isBmpCodePoint only checks whether the value fits in 16 bits, not whether it is a valid Unicode scalar value; surrogates are reserved for UTF-16 encoding pairs and casting one to char produces an isolated surrogate that corrupts downstream UTF-8 encoding or text normalization.

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[garydgregory]

Closing; duplicate of #436 , same question.

[rootvector2]

fair enough on the dup. to answer the question here too: you never type the raw character, the converter parses the literal six ascii chars \uD800 as an escape via the startsWith("\\u") branch, so a plain-ascii --opt \uD800 argument is enough to get an unpaired surrogate char back.

[garydgregory]

Reopening: I see your point, thank you for replying. Reviewing again.

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