Show related packages based on description embeddings

Package pages get a "Related packages" section: a nearest neighbor
query against the package's own stored embedding (no external service
call), excluding the package itself. Packages without an embedding
render no section.

app/views/packages/details.slim

@@ -70,6 +70,19 @@
       / #metadata is the anchor for this sticky container.
       = render(partial: 'details_rightbox', locals: {pkg: @package})
 
+  / Semantically similar packages, based on the package's description embedding
+  - if @related_packages.any?
+    #related
+      h3 Related packages
+      .grid-x.grid-margin-x.medium-up-2.large-up-3 data-equalizer=true
+        - @related_packages.each do |pkg|
+          .cell.pkgcard data-equalizer-watch=true
+            a.black href=package_path(name: pkg.name)
+              .text.pkgname
+                = pkg.name
+              .text.small
+                = pkg.description
+
 javascript:
   // Where to send POST requests for uploads
   let ajax_upload_url = '#{{upload_receive_json_path}}';