feat: replace index with foundation smoke test page

- Import BaseLayout, NavBar, and PostHeader components
- PostHeader with title 'Speeding Up Webpack Typescript Incremental Builds by 7x',
  date 2019-08-16, type post, readingTime 12
- Three typography voices in prose-width container:
  - Prose voice: Source Serif 4 at 18px (font-prose)
  - Structure voice: Inter at 14px in secondary color (font-structure)
  - Evidence voice: JetBrains Mono with code-text color, surface-code bg,
    rounded-md, accent-primary left border (font-evidence)
- Add build verification test (tests/verify-smoke-page.mjs)
- npm run build succeeds clean

src/pages/index.astro

@@ -1,17 +1,28 @@
 ---
 import BaseLayout from '../layouts/BaseLayout.astro';
 import NavBar from '../components/NavBar.astro';
+import PostHeader from '../components/PostHeader.astro';
 ---
 
-<BaseLayout title="Crash Test Dev" description="Blog by Ken Chau">
+<BaseLayout title="Crash Test Dev" description="Foundation smoke test">
   <NavBar slot="nav" />
-  <h1 style="font-family: var(--font-prose); font-size: var(--type-2xl); color: var(--text-primary);">
-    Crash Test Dev
-  </h1>
-  <p style="font-family: var(--font-structure); font-size: var(--type-base); color: var(--text-secondary); margin-top: var(--space-2);">
-    Blog by Ken Chau
-  </p>
-  <p style="font-family: var(--font-evidence); font-size: var(--type-sm); color: var(--text-tertiary); margin-top: var(--space-4); background: var(--surface-code); padding: var(--space-1); border-radius: var(--rounded-md);">
-    Tokens and fonts are working.
-  </p>
+  <PostHeader
+    title="Speeding Up Webpack Typescript Incremental Builds by 7x"
+    date={new Date('2019-08-16')}
+    type="post"
+    readingTime={12}
+  />
+  <div style="max-width: var(--width-prose); margin-left: auto; margin-right: auto; display: flex; flex-direction: column; gap: var(--space-4);">
+    <p style="font-family: var(--font-prose); font-size: 18px; color: var(--text-primary); line-height: var(--leading-prose);">
+      This is the prose voice — Source Serif 4 at 18px. It carries the reader through long-form
+      content with a warm, readable serif that feels like ink on parchment.
+    </p>
+    <p style="font-family: var(--font-structure); font-size: 14px; color: var(--text-secondary); line-height: var(--leading-normal);">
+      This is the structure voice — Inter at 14px in secondary color. It handles navigation,
+      labels, metadata, and UI chrome with a clean sans-serif that stays out of the way.
+    </p>
+    <code style="display: block; font-family: var(--font-evidence); font-size: var(--type-sm); color: var(--code-text); background: var(--surface-code); padding: var(--space-2); border-radius: var(--rounded-md); border-left: 3px solid var(--accent-primary);">
+      const buildTime = 7 * previousBuildTime; // evidence voice — JetBrains Mono
+    </code>
+  </div>
 </BaseLayout>