fix(uts39): proper Unicode case folding and restriction levels

[SCKelemen]

Two correctness fixes in confusable detection / identifier safety.

First, Skeleton previously used strings.ToLower which is wrong per UTS #39; it now uses full Unicode case folding (generated casefold_data.go from CaseFolding.txt) with the correct transform order and a defensive fixed-point cap that panics on non-convergence instead of silently truncating.

Second, GetRestrictionLevel is fixed so the higher restriction levels are reachable (affects IsSafeIdentifier): the old code had a dead duplicate single-script branch and collapsed everything else to ModeratelyRestrictive, so HighlyRestrictive/ModeratelyRestrictive were not properly distinguished. It now implements the UAX #39 §5.2 Table 1 script-set rules (Latn+Jpan / Latn+Hanb / Latn+Kore as subset checks for HighlyRestrictive, Latin + one non-Cyrillic/Greek script for ModeratelyRestrictive).

Includes the case-fold generator and regression tests.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fold confusable targets before returning skeletons

For inputs whose confusable mapping target is itself case-foldable, this now returns a non-final skeleton because the fixed-point loop only repeats NFD and confusable mapping, not case folding. For example, the checked-in data maps 0 to "O", so Skeleton("0") returns "O" while Skeleton("O") returns "o", making AreConfusable("0", "O") false and violating the idempotence promised by the new comment. The old lowercase-after-mapping path handled these targets; the new full fold needs to be applied after mappings or the data needs to be pre-folded.

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