fix(packet): IPv6 compression for leading-zero addresses

[msimerson]

const toIPv6 = buffer => buffer
  .map(part => (part > 0 ? part.toString(16) : '0'))
  .join(':')
  .replace(/\b(?:0+:){1,}/, ':');

The regex replaces the first run of 0: groups with a single :. For an address that starts with zeros, the resulting string begins with : instead of ::, producing an invalid text representation. Examples:

Real address	toIPv6 output	Correct
::1	:1	::1
::	:0	::
fe80::1	fe80::1 ✓	fe80::1

Parsed AAAA records for loopback, unspecified, and other leading-zero addresses will display incorrectly, and round-tripping them through fromIPv6 → toIPv6 will fail.