feat: add /pricing and /changelog SEO pages

- Pricing page with full comparison table, feature matrix, FAQ, JSON-LD Product schema
- Changelog page with all versions v0.1.0-v0.6.0, JSON-LD Blog schema
- 301 redirects for clean URLs
- Added to sitemap.xml
- Pricing in main nav, changelog in footer
- 14 new tests (171 total)

public/index.html

@@ -240,7 +240,7 @@ footer{border-top:1px solid var(--border);padding:48px 24px 32px;background:var(
     <div class="nav-links">
       <a href="#features">Features</a>
       <a href="#playground">Try It Free</a>
-      <a href="#pricing">Pricing</a>
+      <a href="/pricing">Pricing</a>
       <a href="#docs">API Docs</a>
       <a href="/docs" target="_blank">Swagger</a>
       <a href="/usage">Usage</a>
@@ -738,6 +738,7 @@ screenshot = snap.<span class="fn">capture</span>(
       <a href="#docs">Quick Start</a>
       <a href="/health">Status</a>
       <a href="/usage">Usage Dashboard</a>
+      <a href="/changelog">Changelog</a>
     </div>
     <div class="footer-col">
       <h5>Legal</h5>