External link marker: new tab + icon on links leaving the blog

External links in post bodies now open in a new tab and show a
box-with-arrow marker icon, so readers can tell at a glance which
links leave the site.

- blogex: new Blogex.ExternalLinks.rewrite/1 marks up anchors with
  http(s) hrefs at compile time with target="_blank", rel="noopener"
  and class="blogex-external" (idempotent, merges into existing
  class/rel values, leaves relative, mailto: and #anchor links alone)
- blogex: Post.build/3 applies the rewrite, so every blog gets it
- app: CSS in .blogex-post-body draws the marker via a mask so it
  inherits the link colour; empty ::after content stays invisible
  to screen readers
- make test now also runs the blogex suite; it is currently red
  with 3 pre-existing blog_integration_test failures caused by the
  vacuous link validator (parked yak
  link-validator-never-sees-compiled-html-...)

app/test/firehose_web/controllers/blog_test.exs

@@ -35,6 +35,52 @@ defmodule FirehoseWeb.BlogTest do
     end
   end
 
+  describe "external links in compiled posts" do
+    # The marker markup is applied at compile time by Blogex.ExternalLinks
+    # (blogex/test/blogex/external_links_test.exs covers the rewrite itself);
+    # these tests verify the real compiled posts make it to the page.
+    defp latest_post_with_links,
+      do: Firehose.EngineeringBlog.get_post!("delayed-gratification-in-ai-summer")
+
+    test "wikipedia link carries target, rel and marker class in compiled body" do
+      post = latest_post_with_links()
+
+      assert post.body =~
+               ~s(<a href="https://en.wikipedia.org/wiki/Attention_Is_All_You_Need" target="_blank" rel="noopener" class="blogex-external">)
+    end
+
+    test "every external anchor in compiled body is marked up" do
+      post = latest_post_with_links()
+
+      anchors = Regex.scan(~r/<a\s[^>]*>/, post.body) |> List.flatten()
+
+      external_anchors =
+        Enum.filter(anchors, &(&1 =~ ~r<href="https?://>))
+
+      assert length(external_anchors) > 1
+
+      assert Enum.all?(external_anchors, fn anchor ->
+               anchor =~ ~s(target="_blank") and
+                 anchor =~ ~s(rel="noopener") and
+                 anchor =~ ~s(class="blogex-external")
+             end)
+    end
+
+    test "internal links in compiled body are untouched" do
+      post = latest_post_with_links()
+
+      assert post.body =~
+               ~s(<a href="/blog/engineering/swim-the-test">Swim the Test</a>)
+    end
+
+    test "rendered post page shows external links opening in a new tab", %{conn: conn} do
+      body =
+        conn |> get("/blog/engineering/delayed-gratification-in-ai-summer") |> html_response(200)
+
+      assert body =~ "target=\"_blank\" rel=\"noopener\" class=\"blogex-external\""
+    end
+  end
+
   describe "input validation" do
     test "GET /blog/nonexistent returns 404", %{conn: conn} do
       assert conn |> get("/blog/nonexistent") |> html_response(404)