Fix compile-time link validation and add tag page links

- Restore HTML link extraction in LinkValidator (removed in a83634d
  under the false premise that post bodies are raw markdown; they
  are HTML rendered by NimblePublisher at compile time). The missing
  regex made extract_links/1 find zero links, silently disabling
  compile-time validation.
- Support /blog/{blog_id}/tag/{tag} links: validate blog ID,
  require non-empty tag (tags are user-defined, e.g. pi.dev).
- Fix invalid links in two posts: tag/Pi.dev -> tag/pi.dev,
  2026-07-13-synthetic-tdd.md -> synthetic-tdd.
- Fix test warnings: use Plug.Test deprecation, unused import,
  runtime-defined TestBlogValid module.
- Add regression tests for HTML extraction and tag page links.

blogex/test/blogex/blog_integration_test.exs

@@ -59,7 +59,7 @@ defmodule Blogex.BlogIntegrationTest do
       """)
 
       [{TestBlogValid, _bytecode}] = Code.compile_file(tmp_file, __ENV__.file)
-      assert TestBlogValid.title() == "Test Blog"
+      assert apply(TestBlogValid, :title, []) == "Test Blog"
 
       File.rm!(tmp_file)
     end

blogex/test/blogex/link_validator_test.exs

@@ -13,6 +13,22 @@ defmodule Blogex.LinkValidatorTest do
              ]
     end
 
+    test "extracts internal blog links from HTML body" do
+      body =
+        ~s(<p>Check out <a href="/blog/engineering/hello-world">hello world</a> and <a href="/blog/releases/v1-0-0">release v1</a>.</p>)
+
+      assert LinkValidator.extract_links(body) == [
+               "/blog/engineering/hello-world",
+               "/blog/releases/v1-0-0"
+             ]
+    end
+
+    test "extracts tag page links from HTML body" do
+      body = ~s(<p>See <a href="/blog/engineering/tag/pi.dev">pi.dev posts</a>.</p>)
+
+      assert LinkValidator.extract_links(body) == ["/blog/engineering/tag/pi.dev"]
+    end
+
     test "ignores external links" do
       body = "See [GitHub](https://github.com) and [internal](/blog/engineering/post)."
 
@@ -132,6 +148,25 @@ defmodule Blogex.LinkValidatorTest do
       assert LinkValidator.validate_link("/blog/engineering/post#section") == :ok
     end
 
+    test "allows tag page links" do
+      assert LinkValidator.validate_link("/blog/engineering/tag/pi.dev") == :ok
+      assert LinkValidator.validate_link("/blog/releases/tag/v1") == :ok
+    end
+
+    test "allows tag page links with fragments" do
+      assert LinkValidator.validate_link("/blog/engineering/tag/pi.dev#top") == :ok
+    end
+
+    test "rejects tag page link with empty tag" do
+      assert LinkValidator.validate_link("/blog/engineering/tag/") ==
+               {:error, "empty tag in tag link"}
+    end
+
+    test "rejects unknown blog ID in tag page link" do
+      assert LinkValidator.validate_link("/blog/unknown/tag/pi.dev") ==
+               {:error, "unknown blog ID: unknown"}
+    end
+
     test "rejects non-blog path" do
       assert LinkValidator.validate_link("/about") ==
                {:error, "not a blog link: /about"}

blogex/test/blogex/registry_test.exs

@@ -1,7 +1,6 @@
 defmodule Blogex.RegistryTest do
   use ExUnit.Case
 
-  import Blogex.Test.PostBuilder
   alias Blogex.Registry
 
   defmodule AlphaBlog do

blogex/test/blogex/router_test.exs

@@ -1,6 +1,6 @@
 defmodule Blogex.RouterTest do
   use ExUnit.Case
-  use Plug.Test
+  import Plug.Test
 
   import Blogex.Test.PostBuilder
   alias Blogex.Test.FakeBlog