Constraint: Charset (UTF-8 validity)

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Goal

Add a constraint inspired by Symfony Charset — verify that a string is correctly encoded in a given character encoding.

Reference

• Symfony: Symfony\Component\Validator\Constraints\Charset (via iconv).

Constraints

• No new dependencies — reasonable first iteration scope: UTF-8 via utf8.ValidString / runes; other encodings only if they can be checked with stdlib without heavy tables.

Scope

• Clearly document limitations relative to Symfony in godoc; tests for invalid UTF-8.

Complexity

2 / 5.

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Migrated from Linear: IGO-42
Priority: High
Estimate: 2 points