feat: category neighbours on the post page

The post page links previous_in_category and next_in_category (the
Journal's opt-in scored related mode) as server-rendered links with the
title in the anchor text when the Journal answers one, else the
site-wide previous_post and next_post (postNeighbours). Types, a fixture
post and expectation lines for both cases.

testing/expect-acme.txt

@@ -134,3 +134,10 @@
 200 /blog/tag/release count=application/ld+json:1 contains=ItemList
 200 /blog/author/jane-doe count=application/ld+json:1 contains=ItemList
 200 /blog/hello-acme notcontains=ItemList
+
+# Category neighbours (the Journal's opt-in related_mode scored): the post
+# answer's previous_in_category and next_in_category render as server-side
+# links instead of the site-wide neighbours; without them the site-wide
+# previous_post and next_post stay.
+200 /blog/heading-on contains=href="/blog/acme-note-04" class="journal-post__previous" rel="prev"><span>Previous post</span> Acme Note 04</a> contains=href="/blog/acme-note-06" class="journal-post__next" rel="next"><span>Next post</span> Acme Note 06</a>
+200 /blog/heading-off contains=href="/blog/heading-on" class="journal-post__next" rel="next"><span>Next post</span> Heading On Sample</a> notcontains=journal-post__previous

testing/fixtures/acme/posts.json

@@ -497,7 +497,17 @@
     "translations": {},
     "created_at": "2025-12-20T09:00:00+00:00",
     "updated_at": "2025-12-20T09:00:00+00:00",
-    "content_h1_to_h2": true
+    "content_h1_to_h2": true,
+    "previous_in_category": {
+      "id": 4,
+      "title": "Acme Note 04",
+      "slug": "acme-note-04"
+    },
+    "next_in_category": {
+      "id": 6,
+      "title": "Acme Note 06",
+      "slug": "acme-note-06"
+    }
   },
   {
     "id": 16,