fromHex validated only that the string was even-length, then used
parseInt(pair, 16) per byte. parseInt returns NaN for a non-hex pair, and a
Uint8Array stores NaN as 0 — so fromHex('zzzz') silently returned [0, 0]
instead of failing. Add a hex-charset check (allowing empty input) so bad
input throws. Add bytes.test.ts covering valid decode, round-trip, odd-length
and non-hex cases.
