feat(blog): build per-post searchText, wire to PostRow and featured

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

src/components/molecules/BlogSearch.astro

@@ -0,0 +1,3 @@
+---
+// Stub — real implementation lands in Task 4.
+---

src/pages/blog/index.astro

@@ -5,6 +5,7 @@ import Mark from "~/components/atoms/Mark.astro";
 import PostRow from "~/components/molecules/PostRow.astro";
 import YearHead from "~/components/molecules/YearHead.astro";
 import InkArrow from "~/components/atoms/InkArrow.astro";
+import BlogSearch from "~/components/molecules/BlogSearch.astro";
 import { blog } from "~/lib/urls";
 import { getCollection } from "astro:content";
 
@@ -25,6 +26,32 @@ for (const post of rest) {
   byYear.set(year, bucket);
 }
 const yearGroups = Array.from(byYear.entries()).sort(([a], [b]) => b - a);
+
+// Build the per-post `data-search` string at build time. We concatenate the
+// searchable fields, lowercase, strip a small set of markdown punctuation, and
+// collapse whitespace. Body text is the raw markdown from the content
+// collection entry's `.body` property — MDX expressions that survive end up as
+// literal text, which is harmless for substring matching.
+function buildSearchText(post: (typeof posts)[number]): string {
+  const parts = [
+    post.data.title,
+    post.data.tag,
+    post.data.standfirst,
+    post.data.description,
+    post.body ?? "",
+  ].filter(Boolean) as string[];
+  return parts
+    .join(" ")
+    .toLowerCase()
+    .replace(/[#*_>`\[\]()]/g, " ")
+    .replace(/\s+/g, " ")
+    .trim();
+}
+
+const searchTextById = new Map<string, string>();
+for (const post of posts) {
+  searchTextById.set(post.id, buildSearchText(post));
+}
 ---
 
 <BaseLayout
@@ -53,7 +80,7 @@ const yearGroups = Array.from(byYear.entries()).sort(([a], [b]) => b - a);
 
   {
     featured && (
-      <article class="feature">
+      <article class="feature" data-featured data-search={searchTextById.get(featured.id) ?? ""}>
         <div class="cutframe img">
           <div class="grain" />
           <img class="wm" src="/assets/mark-rings.svg" alt="" />
@@ -91,6 +118,7 @@ const yearGroups = Array.from(byYear.entries()).sort(([a], [b]) => b - a);
               href={blog(`/posts/${post.id}/`)}
               deck={post.data.standfirst}
               tag={post.data.tag}
+              searchText={searchTextById.get(post.id) ?? ""}
             />
           ))}
         </>