refactor: fork the URI and URL engines behind static dispatch

[OmarAlJarrah]

Three internal files — the host parser, the IPv4 parser, and the recomposition
serializer — were each a single object that took a profile argument and branched
if (profile.isWhatwg) … else … at runtime to serve both the RFC 3986 (URI) and
the WHATWG (URL) code paths. Every caller of those objects is single-profile, so
that runtime branch was hidden control flow with no benefit.

This forks each of the three into a per-spec unit and lets each caller dispatch
statically:

• Serializer → UriSerializer + UrlSerializer, with the shared authority and
  query/fragment helpers hoisted into SerializeShared.
• Ipv4 → Ipv4Rfc3986 + Ipv4Whatwg; the shared serialization, octet packing,
  and endsInANumber remain on Ipv4.
• HostParser → UriHostParser + UrlHostParser, with the shared bracket helpers
  in HostParsing.

The paired spec units never reference each other — genuinely shared code lives in
the three neutral files above — so a change to the URL path can no longer silently
affect the URI path, and vice versa.

With the three dispatchers gone, the ParseProfile enum had no remaining callers
and is removed. Separately, the shared path model UrlPath — used by the URI path,
the reference resolver, and the normalizer as well as by URL parsing — is renamed
ComponentPath so its name no longer implies it is WHATWG-specific. As a small
cleanup, a duplicate private SLASH constant in UriNormalizer is dropped in favour
of the shared one introduced by the serializer split.

No behaviour change and no public API change: every moved function body is preserved
verbatim, the full existing conformance suite (URL, URI, host, IDNA, serializer,
resolver, IPv4) passes unchanged, the binary-compatibility snapshots are untouched
(apiCheck clean), and there are no generated-table or codegen edits.