fix(zarith): raise ArgumentError on malformed input

[vhf]

• decode("80") returned 0 (continuation bit set, no following byte)
• decode("ff") returned -63 (instead of raising)
• consume("") raised MatchError from hex_to_dec("")
• consume("a1d22") (odd hex length) raised FunctionClauseError

Now consume/1 validates upfront (non-empty, even-length hex) and find_int + read_next raise on truncation (continuation bit set with no following byte). read/1 now branches on the halt bit explicitly so single-byte zarith inputs still terminate cleanly.