fix(ipaddr): strict IPv6 validation — reject :: with zero compression and hextets >4 hex digits - #206

Closes #205

Changes

lib/ipaddr.js

1. colonCount > parts → colonCount >= parts — reject IPv6 addresses where :: compresses zero parts (e.g. 1:2:3:4:5:6:7:8::, ::1:2:3:4:5:6:7:8), per RFC 4291
2. Hextet length check — reject hextets longer than 4 hexadecimal digits (e.g. 00000::1), per RFC 4291 section 2.2

test/ipaddr.test.js

• Added test: "rejects IPv6 parts longer than four hexadecimal digits"
• Added test coverage for IPv4-embedded IPv6 with overlong hextets
• Added test: "rejects compression of zero IPv6 parts"
• Added regression test to ensure compression of exactly one IPv6 part remains valid