feat(structure): compact archive + tag pages + hero/featured, and fix site search

Found a real bug while verifying the nav search: /pagefind/pagefind.js is an ES
module that defines no window.Pagefind, but Search.astro loaded it with a classic
<script src> and read window.Pagefind — so every query answered "Search is
unavailable." (confirmed live before the fix). Now a dynamic import; the specifier
must be a variable or Rollup resolves it at build time and the build fails.

Measured before/after in a real browser:

- archive /posts/ + /zh/posts/: 18,220px -> 3,788px (71 rows at 44px, grouped by
  year) with a client-side filter (title/tags/description text, category chips,
  14 tag chips, zero requests). Tested: "nginx" -> 6 rows, chip devops -> 28.
- tag pages for tags with >= 3 posts. Measured 235 distinct tags (an earlier brief
  claimed 74), 156 used exactly once — pages only where a topic exists: 40 tags ->
  80 pages, build 165 -> 245 pages. Pills link only when a page exists.
- homepage + ZH homepage: hero (288px) with the real scale claim, three CTAs, a
  fixed Featured 4 (scale -> automation -> observability -> a delivered result),
  Latest 10 -> 6. ZH hero had linked "关于我" to the English /about/.
- long code blocks collapse above 420px with an Expand button + gradient, copy
  button still copies the whole block (verified on the 1010px block).
- search is visible ("⌕ Search /" pill, "/" plus the existing Ctrl/Cmd+K) and the
  zero-result screen offers topic chips instead of a dead end.

Docs: project-state B3 entry, ops-runbook Pagefind gotcha.

src/lib/tags.ts

@@ -0,0 +1,43 @@
+import type { CollectionEntry } from 'astro:content';
+
+type Post = CollectionEntry<'posts'>;
+
+/**
+ * Tag pages are generated only for tags that actually carry a topic.
+ *
+ * Measured 2026-10-16: 235 distinct tags across the posts, of which **156 appear
+ * exactly once** and 40 appear >= 3 times. Generating a page per tag would create
+ * 156 near-empty pages (thin content) and no navigation value; the threshold below
+ * is the line between "a topic" and "a word that appeared in one post".
+ */
+export const TAG_MIN_POSTS = 3;
+
+/** Count how many posts carry each tag. */
+export function tagCounts(posts: Post[]): Map<string, number> {
+  const counts = new Map<string, number>();
+  for (const p of posts) {
+    for (const t of p.data.tags) counts.set(t, (counts.get(t) ?? 0) + 1);
+  }
+  return counts;
+}
+
+/** Tags that get their own page, most-used first (stable, no locale drift). */
+export function tagPageNames(posts: Post[]): string[] {
+  return [...tagCounts(posts).entries()]
+    .filter(([, n]) => n >= TAG_MIN_POSTS)
+    .sort((a, b) => b[1] - a[1] || a[0].localeCompare(b[0]))
+    .map(([t]) => t);
+}
+
+/** Where a tag links to in a given language. */
+export function tagHref(tag: string, lang: 'en' | 'zh'): string {
+  return lang === 'zh' ? `/zh/tags/${tag}/` : `/tags/${tag}/`;
+}
+
+/**
+ * Tag pills: a tag with a page becomes a link, a one-off tag stays plain text.
+ * Without this split the pills look identical whether or not they go anywhere.
+ */
+export function tagLink(tag: string, counts: Map<string, number>, lang: 'en' | 'zh'): string | null {
+  return (counts.get(tag) ?? 0) >= TAG_MIN_POSTS ? tagHref(tag, lang) : null;
+}