Replace posts/001/slug nesting with issue taxonomy

Use Hugo's `issue = "issues"` taxonomy so each post carries
`issues: ["01"]` in frontmatter. Issue pages live at /issues/01/
with a dedicated _index.md holding title, season, and description.
The /issues/ archive page uses data/issues.json for forthcoming
entries that have no posts yet.

Layouts:
- layouts/issues/list.html → plate grid for a single issue term
- layouts/issues/terms.html → issues archive (new)
- _default/single.html → scopes grid to issue term page, back-link to /issues/01/
- baseof.html → ribbon links to /issues/01/, JSON island includes issue field
- partials/header.html → Issues link → /issues/

JS:
- lbClose() navigates to /issues/${issueId}/ on a single-post page
- lightbox meta panel shows linked Issue fact

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

layouts/baseof.html

@@ -15,7 +15,7 @@
   {{- partial "lightbox.html" . }}
 
   <div class="ribbon" id="ribbon">
-    <span>Roux № {{ .Site.Params.issueNumber }} — out now. <a href="/categories/gothic/">See the plates →</a></span>
+    <span>Roux № {{ .Site.Params.issueNumber }} — out now. <a href="/issues/01/">See the plates →</a></span>
     <button id="ribbonClose" aria-label="Dismiss">
       <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 6 6 18"/><path d="m6 6 12 12"/></svg>
     </button>
@@ -37,6 +37,7 @@
       {{- $post := dict
           "id"    .Params.plate
           "slug"  .Params.slug
+          "issue" (index (.Params.issues | default (slice "01")) 0)
           "title" .Title
           "description" .Params.description
           "categories" .Params.categories