Frontmatter nav consistency: add missing up links to 36 pages

Every front-matter page now has a 3-link chapter-nav (prev/up/next).
Fixed nav formatting in section-fm.5.html.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: apartsin

_audit_frontmatter.py

@@ -339,6 +339,7 @@ <h2>What Comes Next</h2>
 
     <nav class="chapter-nav">
         <a class="prev" href="../toc.html">Table of Contents</a>
+        <a class="up" href="../toc.html">Contents</a>
         <a class="next" href="section-fm.1.html">FM.1: Introduction</a>
     </nav>
 

_fix_frontmatter_nav.py

@@ -231,11 +231,11 @@ <h2>Why This Matters</h2>
         see the <a href="../team.html">Writing Team</a> reference page.
     </p>
 
-<nav class="chapter-nav">
-    <a href="section-fm.4.html" class="prev">FM.4: How to Use This Book</a>
-    <a href="index.html">Front Matter</a>
-    <a href="../part-1-foundations/module-00-ml-pytorch-foundations/index.html" class="next">Chapter 00: ML &amp; PyTorch Foundations</a>
-</nav>
+    <nav class="chapter-nav">
+        <a class="prev" href="section-fm.4.html">FM.4: How to Use This Book</a>
+        <a class="up" href="index.html">Front Matter</a>
+        <a class="next" href="../part-1-foundations/module-00-ml-pytorch-foundations/index.html">Chapter 00: ML &amp; PyTorch Foundations</a>
+    </nav>
 
     <footer><p>Fifth Edition, 2026 &middot; <a href="../toc.html">Contents</a></p></footer>
 </main>