fix: hex_or_base64 digit recognition

[brunocapelao]

Summary

Fix a bug in the hex_or_base64() function where digits (0-9) were not recognized as valid lowercase hex characters.

Problem

The original code used is_ascii_lowercase() to check for lowercase hex characters:

s.bytes().all(|b| b.is_ascii_hexdigit() && b.is_ascii_lowercase())

However, is_ascii_lowercase() returns false for digits 0-9, causing valid lowercase hex strings like "0a1b2c3d" to be incorrectly parsed as base64 instead of hex.

Solution

Use explicit pattern matching for valid lowercase hex characters (0-9, a-f):

s.bytes().all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'f'))

Testing

This bug was discovered while using hal-simplicity for Simplicity transactions on Liquid Testnet. After the fix, hex strings containing digits are correctly parsed.