fix: normalize BASE_URL trailing slash in post links and RSS feed

The Astro BASE_URL (/crashtestdev) lacked a trailing slash, causing
post URLs to render as /crashtestdevposts/... instead of
/crashtestdev/posts/... Breaking article card navigation and RSS feed
links.

Applied the same .replace(/\/?$/, '/') normalization pattern already
used in BaseLayout.astro to:
- ArticleCard.astro (card link hrefs)
- NavBar.astro (isActive path matching)
- rss.xml.ts (item link generation)

Also adds final-verification.test.mjs with 31 tests covering:
- Clean build output structure
- Index page cards, shadows, hover states, URL format
- Post page NavBar active state, PostHeader centering
- Three typography voices (prose/structure/evidence)
- Code block breakout width, warm background, terracotta border
- Design token color compliance
- Responsive behavior (nav stacking, title shrinking, gutter respect)
- RSS feed validity and URL correctness

src/pages/rss.xml.ts

@@ -4,7 +4,7 @@ import type { APIContext } from 'astro';
 
 export async function GET(context: APIContext) {
   const posts = await getCollection('posts');
-  const base = import.meta.env.BASE_URL;
+  const base = import.meta.env.BASE_URL.replace(/\/?$/, '/');
 
   const sortedPosts = posts.sort(
     (a, b) => b.data.date.getTime() - a.data.date.getTime()