Implement a function that measures the longest prefix made only of characters from an allowed set.

• The function receives two null-terminated strings: text and allowed.
• Starting at the first character of text, count characters while each one appears somewhere in allowed.
• Stop as soon as a character from text is not present in allowed.
• Return the number of accepted leading characters.
• Return 0 when text is empty or when its first character is not allowed.
• Do not call library span/search helpers.

Implement a function that measures the longest prefix made only of characters from an allowed set.

• The function receives two null-terminated strings: text and allowed.
• Starting at the first character of text, count characters while each one appears somewhere in allowed.
• Stop as soon as a character from text is not present in allowed.
• Return the number of accepted leading characters.
• Return 0 when text is empty or when its first character is not allowed.
• Do not call library span/search helpers.