Eliminate redundant comparisons by following the ASCII sequence

[spirinvladimir]

Optimize Hexadecimal Validation Logic

Problem: Original logic performed redundant comparisons regardless of ASCII value ordering.

Solution: Restructured validation to follow ASCII sequence (0-9: 48-57, A-F: 65-70, a-f: 97-102):

if (ch < '0') return 0;           // < 48: Invalid
if (ch <= '9') continue;          // 48-57: Valid digits
if (ch < 'A') return 0;           // 58-64: Invalid gap  
if (ch <= 'F') continue;          // 65-70: Valid A-F
if (ch < 'a') return 0;           // 71-96: Invalid gap
if (ch <= 'f') continue;          // 97-102: Valid a-f
return 0;                         // > 102: Invalid

Benefits:

• Eliminates redundant range checks
• Reduces comparisons from 6 to 2-3 per character
• Early exit on invalid characters
• Follows natural ASCII ordering for better branch prediction

[mhw0]

Hi @spirinvladimir, merged. Thanks for the PR!