feat(net/unstable): classify IP addresses by IANA special-purpose registry

[tomas-zijdemans]

Adds classifyIP() plus isLoopback(), isPrivate(), isLinkLocal(),
isMulticast() and isUnspecified(). Step 2 of #7315, on top of the parsers from
#7316.

Lookup is a longest-prefix match over a table transcribed from the two IANA
special-purpose registries, and the nesting is why. 192.0.0.9 and
192.0.0.10 are globally reachable inside a reserved 192.0.0.0/24.
2001::/23 is reserved "unless allowed by a more specific allocation", with
seven reachable allocations inside it. A chain of ifs gets that right only if
every exception is hand-written in the right place.

IPv4-mapped addresses are unmapped before lookup, because ::ffff:127.0.0.1 is
127.0.0.1. Tunnels are not: 6to4, NAT64, Teredo and the deprecated ::/96
are a route to an address rather than that address, so they classify by their
own block. Embedded-IPv4 extraction and isGloballyReachable() wait for a
caller who needs them. Next from me is maskIP() and formatIP(), which is
what @std/rate-limit calls to key a client by its IPv6 allocation rather than
its address.

Look hardest at the two tables. I diffed every row against the registry CSVs,
checked that every Globally-Reachable-True row comes back global, and checked
that every row is a canonical CIDR, because parseSubnet() masks host bits
silently. Three rows come from RFCs instead of the registries and are cited in
place: 224.0.0.0/4, ff00::/8, and fec0::/10, which IANA dropped when RFC
3879 deprecated site-local. I also ran the predicates against Python's
ipaddress over 16,244 addresses: loopback, link-local, multicast and
unspecified agree everywhere.

Two caller notes. url.hostname keeps IPv6 brackets, so you strip them
yourself (documented, with a runnable example). Unallocated space like
4000::/3 comes back global, the same simplification Go's IsGlobalUnicast
makes.

I used Claude Code to help investigate and write this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.05%. Comparing base (01fe755) to head (481c703).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7324    +/-   ##
========================================
  Coverage   95.04%   95.05%            
========================================
  Files         618      619     +1     
  Lines       51746    52110   +364     
  Branches     9402     9462    +60     
========================================
+ Hits        49181    49531   +350     
- Misses       2022     2031     +9     
- Partials      543      548     +5     

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