fix(ans): reject non-hex input in fromHex instead of silently decoding to 0 (#91)

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.

packages/ans/src/bytes.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+import { fromHex, toHex } from './bytes.js';
+
+describe('fromHex', () => {
+  it('decodes valid hex, with and without 0x prefix', () => {
+    expect([...fromHex('00ff10')]).toEqual([0x00, 0xff, 0x10]);
+    expect([...fromHex('0xdeadbeef')]).toEqual([0xde, 0xad, 0xbe, 0xef]);
+    expect([...fromHex('')]).toEqual([]);
+  });
+
+  it('round-trips with toHex', () => {
+    const bytes = Uint8Array.from([0, 1, 127, 128, 255]);
+    expect([...fromHex(toHex(bytes))]).toEqual([...bytes]);
+  });
+
+  it('throws on odd-length input', () => {
+    expect(() => fromHex('abc')).toThrow(/odd-length/);
+  });
+
+  it('throws on non-hex characters instead of silently decoding to 0', () => {
+    // Previously parseInt('zz', 16) === NaN, which a Uint8Array stores as 0,
+    // so fromHex('zzzz') silently returned [0, 0].
+    expect(() => fromHex('zzzz')).toThrow(/invalid hex/);
+    expect(() => fromHex('00gg')).toThrow(/invalid hex/);
+    expect(() => fromHex('0xnothex')).toThrow(/invalid hex/);
+  });
+});