Implemented the `is_valid_utf8_char` procedure to safely validate UTF-8
sequences and return their byte length (1-4, or 0 if invalid).
This routine implements strict Unicode compliance checks, including:
- Rejection of overlong encodings (e.g., checking 0xC0/0xC1, and strict
bounds for 0xE0/0xF0).
- Prevention of surrogate half decoding (restricting 0xED bounds).
- Enforcement of the maximum Unicode scalar value limit (U+10FFFF).
- Safe handling of null-terminators and truncated sequences.
This provides a secure foundation for upgrading the codepoint and
grapheme counting functions in upcoming commits.
Swarnadeep Paul
05ecfe005c