std.utf

valid_seq(seq) -> ok

Validate a complete UTF-8 multi-byte sequence

Checks that seq is a well-formed 2–4 byte UTF-8 sequence. Verifies the leading byte with valid_b1 and checks that all continuation bytes are in range (0x80–0xBF). Also enforces the Unicode-mandated exclusions:

• 0xE0 sequences must have a second byte ≥ 0xA0 (no overlong 3-byte)
• 0xED sequences must have a second byte ≤ 0x9F (no surrogates)
• 0xF0 sequences must have a second byte ≥ 0x90 (no overlong 4-byte)
• 0xF4 sequences must have a second byte ≤ 0x8F (no out-of-range)

Does not accept single-byte (ASCII) input; use valid for full strings.