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/pages/index.astro

@@ -1,8 +1,10 @@
 ---
 import BaseLayout from '../layouts/BaseLayout.astro';
+import NavBar from '../components/NavBar.astro';
 ---
 
 <BaseLayout title="Crash Test Dev" description="Blog by Ken Chau">
+  <NavBar slot="nav" />
   <h1 style="font-family: var(--font-prose); font-size: var(--type-2xl); color: var(--text-primary);">
     Crash Test Dev
   </h1>