Autoload Resolv instead of requiring it eagerly

[tas50]

Problem

net/http requires resolv at the top of the file, but the only thing it uses from that library is two regexp constants, on a single line:

# Server Name Indication (SNI) RFC 3546/6066
case @address
when Resolv::IPv4::Regex, Resolv::IPv6::Regex
  # don't set SNI, as IP addresses in SNI is not valid

That line lives in the TLS branch of #connect, so resolv isn't needed until an HTTPS connection is opened — and not at all for plain HTTP, or for the many libraries that require 'net/http' to expose optional functionality and never connect.

   81.54  net/http
   59.10    net/protocol
   54.09      socket
    9.78    uri
    6.33    resolv        <-- for two regexps
    0.77    zlib

Fix

-require 'resolv'
 autoload :OpenSSL, 'openssl'
+autoload :Resolv, 'resolv'

autoload rather than a require inside #connect, for two reasons:

1. It matches what the very next line of this file already does for OpenSSL, so it's the file's existing idiom rather than a new pattern.
2. It's backward compatible. Resolv stays resolvable, so anything downstream that relied on require 'net/http' defining it keeps working — the load just happens on first reference instead of eagerly. An inline require would have removed the constant from the post-require namespace.

It also costs nothing per connection, which an inline require in connect would not.

Measurements

Ruby 4.0.6 (arm64-darwin), best of seven runs:

	require 'net/http'	files loaded
before	76.94 ms	33
after	70.54 ms	30
	−6.40 ms (8%)	−3

Verified directly:

require 'net/http'          -> resolv loaded? NOT loaded
Resolv::IPv4::Regex         -> Regexp, resolv now loaded? true
Net::HTTP.get_response(URI("https://example.com/")) -> 200

Tests

bundle exec rake test: 201 tests, 0 failures, unchanged (204 with the additions).

The three added tests cover that requiring net/http does not load resolv, that Resolv::IPv4::Regex still resolves afterwards, and that referencing it pulls the library in. The first fails against the previous code.

The remaining net/http load cost is socket (54 ms via net/protocol) and uri (10 ms), both of which it genuinely needs — this was the only removable item.