runtime(cruby): the overlay Request answers #host

There are two `ActionDispatch::Request` implementations — the shared
transpilable one (`runtime/ruby/action_dispatch/request.rb`) and the
CRuby overlay over the CGI env, which is the one `main.rb` actually
requires. The shared one has always had `host`; the overlay built the
same string INSIDE `base_url` and exposed nothing.

That made `Rails.application.domain` — the framework default every
`_url` helper grounds against, added in 7a09ba9 as
`ActionController::Current.request.host` — raise NoMethodError on any
app that does not define `domain` itself. campfire's message row links
each message by `room_at_message_url`, so it was every message.

`base_url` now composes from it rather than repeating the fetch, so
the two cannot drift.

Verified over the batch: cargo test (all green), compare ruby 7/7,
smoke ruby 6/6, crystal 3/3 + rust 2/2 + spinel 1/1. The lobsters emit
differs in exactly this one runtime file and nowhere else.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: rubys

runtime/spinel/scaffold/ruby_overlay/runtime/action_dispatch_request.rb

@@ -62,9 +62,19 @@ def original_url = "#{base_url}#{fullpath}"
     # (campfire stores it as the post-login return path).
     def url = original_url
 
+    # Host (with port when the client sent one), as Rails reports it.
+    # The shared `ActionDispatch::Request` has always carried this; the
+    # overlay twin did not, and `Rails.application.domain` — the
+    # framework default every `_url` helper grounds against — reads it.
+    # So an app with no `domain` of its own raised NoMethodError on its
+    # first absolute URL: campfire's message row, which links each
+    # message by `room_at_message_url`.
+    def host
+      @env["HTTP_HOST"] || @env["SERVER_NAME"] || "localhost"
+    end
+
     def base_url
       scheme = @env["HTTPS"] == "on" ? "https" : "http"
-      host = @env["HTTP_HOST"] || @env["SERVER_NAME"] || "localhost"
       "#{scheme}://#{host}"
     end