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/_default/single.html

@@ -1,13 +1,20 @@
 {{ define "main" }}
-{{/* Individual post page — grid renders in background, JS opens lightbox for this slug */}}
+{{- $issue    := index (.Params.issues | default (slice "01")) 0 }}
+{{- $issueURL := printf "/issues/%s/" $issue }}
 <section class="hero" id="hero">
-  <div class="hero__eyebrow">{{ index (.Params.categories | default (slice "Plate")) 0 }}</div>
+  <div class="hero__eyebrow">
+    № {{ $issue }} · {{ index (.Params.categories | default (slice "Plate")) 0 }}
+  </div>
   <h1 class="hero__title"><em>{{ .Title }}</em></h1>
-  <p class="hero__lede">{{ .Params.description }} <a href="/" style="border-bottom:1px solid currentColor">Return to archive →</a></p>
+  <p class="hero__lede">
+    {{ .Params.description }}
+    <a href="{{ $issueURL }}" style="border-bottom:1px solid currentColor">Return to issue →</a>
+  </p>
 </section>
 
 <section class="grid" id="grid" data-density="default">
-  {{- $posts := where .Site.RegularPages "Section" "posts" }}
+  {{- $termPage := $.Site.GetPage (printf "/issues/%s" $issue) }}
+  {{- $posts := cond (ne $termPage nil) $termPage.Pages .Site.RegularPages }}
   {{- range $i, $p := $posts }}
     {{- $img := $p.Resources.GetMatch "*.png" }}
     <a class="card"