feat: See Also and Linked From sections on posts

- `backlinksWith` filter scans raw source files to find posts that
  link to the current post (backlinks), with per-build caching.
- `postByUrl` filter looks up a post by absolute URL for title display.
- post.njk: "See Also" renders resolved `related` URLs with titles;
  "Linked From" lists backlinks computed at build time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_includes/layouts/post.njk

@@ -338,6 +338,40 @@ withBlogSidebar: true
 </article>
 
 
+{# See Also — related posts from frontmatter (resolved wikilinks → URLs) #}
+{% if related and related.length > 0 %}
+<section class="post-related mt-8 pt-6 border-t border-surface-200 dark:border-surface-700">
+  <h2 class="text-xs font-semibold text-surface-500 dark:text-surface-400 uppercase tracking-widest mb-3">See Also</h2>
+  <ul class="space-y-1.5 list-none p-0 m-0">
+    {% for relUrl in related %}
+    {% set _relPost = collections.posts | postByUrl(relUrl) %}
+    <li>
+      <a href="{{ relUrl }}" class="text-sm text-accent-700 dark:text-accent-300 hover:underline">
+        {{ _relPost.data.title if _relPost else relUrl }}
+      </a>
+    </li>
+    {% endfor %}
+  </ul>
+</section>
+{% endif %}
+
+{# Linked From — posts that link to this one, computed from raw source files #}
+{% set _backlinks = collections.posts | backlinksWith(page.url) %}
+{% if _backlinks and _backlinks.length > 0 %}
+<section class="post-backlinks mt-6 pt-6 border-t border-surface-200 dark:border-surface-700">
+  <h2 class="text-xs font-semibold text-surface-500 dark:text-surface-400 uppercase tracking-widest mb-3">Linked From</h2>
+  <ul class="space-y-1.5 list-none p-0 m-0">
+    {% for post in _backlinks %}
+    <li>
+      <a href="{{ post.url }}" class="text-sm text-accent-700 dark:text-accent-300 hover:underline">
+        {{ post.data.title or "Untitled" }}
+      </a>
+    </li>
+    {% endfor %}
+  </ul>
+</section>
+{% endif %}
+
 {# Comments section #}
 {% include "components/comments.njk" %}
 

eleventy.config.js

@@ -1094,6 +1094,31 @@ export default function (eleventyConfig) {
     return stages[stage] || null;
   });
 
+  // Backlinks — find all published posts whose raw source links to the given URL path.
+  // Reads each file once per build and caches it to avoid repeated disk I/O.
+  {
+    const contentCache = new Map();
+    eleventyConfig.addFilter("backlinksWith", function (posts, currentUrl) {
+      const fullUrl = `${siteUrl}${currentUrl}`;
+      return (posts || []).filter((post) => {
+        if (post.url === currentUrl) return false;
+        let content = contentCache.get(post.inputPath);
+        if (content === undefined) {
+          try { content = readFileSync(post.inputPath, "utf-8"); }
+          catch { content = ""; }
+          contentCache.set(post.inputPath, content);
+        }
+        return content.includes(fullUrl);
+      });
+    });
+  }
+
+  // Look up a post by its absolute blog URL — used to show titles in related-links lists.
+  eleventyConfig.addFilter("postByUrl", function (posts, url) {
+    const path = url.replace(siteUrl, "");
+    return (posts || []).find((p) => p.url === path || p.url === `${path}/`);
+  });
+
   // Strip garden/* tags from a category list so they don't render as
   // plain category pills alongside the garden badge.
   eleventyConfig.addFilter("withoutGardenTags", (categories) => {