feat: add NavBar component with site name, Posts link, and responsive layout

Created NavBar.astro component with:
- Site name link 'Crash Test Dev' and Posts nav link with active state detection
- aria-label='Site navigation' and aria-current='page' accessibility attributes
- Responsive stacking at 599px breakpoint
- Design token-based styling (structure font, border-subtle, etc.)
- Updated index.astro to include NavBar in nav slot
- Added comprehensive build-output tests (24 assertions)

Generated with Amplifier
Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

src/components/NavBar.astro

@@ -0,0 +1,98 @@
+---
+const currentPath = Astro.url.pathname;
+const base = import.meta.env.BASE_URL;
+
+const navLinks = [
+  { label: 'Posts', href: base },
+];
+
+function isActive(href: string): boolean {
+  return currentPath === href || currentPath.startsWith(base + 'posts/');
+}
+---
+
+<nav aria-label="Site navigation">
+  <div class="nav-inner">
+    <a href={base} class="site-name">Crash Test Dev</a>
+    <div class="nav-links">
+      {navLinks.map((link) => (
+        <a
+          href={link.href}
+          class:list={['nav-link', { active: isActive(link.href) }]}
+          aria-current={isActive(link.href) ? 'page' : undefined}
+        >
+          {link.label}
+        </a>
+      ))}
+    </div>
+  </div>
+</nav>
+
+<style>
+  nav {
+    border-bottom: 1px solid var(--border-subtle);
+    position: static;
+  }
+
+  .nav-inner {
+    max-width: var(--width-container);
+    margin-left: auto;
+    margin-right: auto;
+    padding-left: var(--gutter);
+    padding-right: var(--gutter);
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+  }
+
+  .site-name {
+    font-family: var(--font-structure);
+    font-size: var(--type-base);
+    font-weight: 600;
+    color: var(--text-primary);
+    letter-spacing: -0.01em;
+    text-decoration: none;
+  }
+
+  .site-name:hover {
+    color: var(--accent-primary);
+  }
+
+  .nav-links {
+    display: flex;
+    gap: var(--space-4);
+  }
+
+  .nav-link {
+    font-family: var(--font-structure);
+    font-size: var(--type-sm);
+    font-weight: 400;
+    color: var(--text-secondary);
+    text-decoration: none;
+  }
+
+  .nav-link:hover {
+    color: var(--text-primary);
+  }
+
+  .nav-link.active {
+    font-weight: 500;
+    color: var(--text-primary);
+    text-decoration: underline;
+    text-decoration-color: var(--accent-primary);
+    text-decoration-thickness: 2px;
+    text-underline-offset: 6px;
+  }
+
+  @media (max-width: 599px) {
+    .nav-inner {
+      flex-direction: column;
+      gap: var(--space-1);
+      text-align: center;
+    }
+
+    .nav-links {
+      gap: var(--space-3);
+    }
+  }
+</style>