Regex-free IPv4 parser guts (strict four-part + inet_aton)

[kleinron]

Summary

In-tree IPv4 parser guts swap (MIT): regex-free hand scanners for strict four-part + loose/inet_aton + CIDR split. No new deps; no @kleinron/ipv4 import. Public IPv4 API and accept set unchanged; IPv6 transitional regexes that reuse ipv4Part stay.

Motivation / benches

Microbench tables below were measured on @kleinron/ipv4 using the same algorithms (not timed on this fork). Please re-bench here if useful.

Full tables: https://github.com/kleinron/ipv4/blob/main/bench/RESULTS.md
Highlight: isValidFourPartDecimal

API (corpus)	Δ vs ipaddr.js@2.5.0 (neg = faster)
isValidFourPartDecimal mixed / accept / reject	−99.3% / −87.5% / −99.7%
isValid loose mixed / accept / reject	−84.5% / −76.6% / −84.9%
parse strict accept	−76.8%
parse loose accept	−80.5%
parse / parseCIDR reject–mixed	more modest (≈ −6% to −20%)
parseCIDR accept	−51.1%

Test plan

• Existing suite green (70/70 on the fork)
• ESLint clean
• Upstream CI

[whitequark]

Not interested in maintaining this, sorry.