proposal(net): classify IP addresses against the IANA special-purpose registries

I'd like to add IP classification to `@std/net/unstable-ip`, backed by the IANA special-purpose registries. Fix the parsers first.

The current validators accept these inputs, reproduced on this checkout:

| Validator | Invalid inputs returning `true` |
| --- | --- |
| `isIPv4` | `"0x7f.0.0.1"`, `" 1.2.3.4"`, `"1..2.3"`, `"01.002.3.4"` |
| `isIPv6` | `":1"`, `"2001:db8:::1"`, `"::1 "` |

In #6765, 0f-0b [requested URL-standard syntax and parsers that return bytes](https://github.com/denoland/std/pull/6765#pullrequestreview-3031081186). This would follow through on that proposal.

There's demand beyond validation. Better Auth's shared host classifier serves eleven call sites across seven packages, including [SSRF fixes](https://github.com/better-auth/better-auth/pull/9226) and [client metadata fetches](https://github.com/better-auth/better-auth/pull/10126). Go `netip`, Rust `std::net`, and Python `ipaddress` already offer address classification. Potential std consumers include client-IP handling in #7237 and unspecified-address handling in `http/file_server.ts`.

Proposed scope, in separate PRs:

1. Add `parseIPv4` / `parseIPv6`, returning 4 / 16 bytes as `Uint8Array`, or `undefined` for invalid input. Share parsing with the validators and subnet matchers.
2. Add `classifyIP(): IPAddressKind | undefined`, plus `isLoopback`, `isPrivate`, `isLinkLocal`, `isMulticast`, and `isUnspecified`. Kinds: loopback, unspecified, private, link-local, shared, documentation, benchmarking, multicast, broadcast, reserved, global.
3. Add `embeddedIPv4` for 6to4, NAT64's well-known prefix, Teredo, and deprecated IPv4-compatible addresses. Add `isGloballyReachable` once the handling of embedded addresses and IANA's N/A entries is defined and tested.
4. Use the helpers in std where they simplify existing code, starting with the file server or rate limiter.

Keep this in `net/unstable_ip.ts`, under the existing entry point. Use longest-prefix matching against the [IPv4](https://www.iana.org/assignments/iana-ipv4-special-registry/) and [IPv6](https://www.iana.org/assignments/iana-ipv6-special-registry/) registries, with RFC-defined multicast ranges. Exceptions matter: `192.0.0.9` and `.10` are globally reachable within a non-global `/24`.

Unmap IPv4-mapped IPv6 before classification, so `::ffff:127.0.0.1` classifies as loopback. Keep tunnel extraction separate from classification of the outer address.

Inputs are bare IP literals: no ports, brackets, whitespace trimming, zone IDs, or hostname classification. URL callers normalize through `URL` and remove IPv6 hostname brackets before passing the address. Global reachability describes address ranges. It does not resolve DNS, prevent rebinding, or check redirects.

For IPv4, I propose the URL standard's [strict dotted-quad syntax](https://url.spec.whatwg.org/#host-writing): four decimal octets, no leading zeros. Should we use that grammar, or the URL parser's permissive grammar accepting hex, octal, and shorthand?