build: content-hash cache-busting for local CSS/JS (?v=hash)

Stops deploys being served stale from browser cache — the recurring "changes
do not show up until a hard refresh" problem.

- New tools/cache_bust.py stamps a global content hash onto every local
  <link>/<script> CSS/JS ref AND the @import lines in css/main.css (so partial
  edits bust too). External (https://) refs are untouched. Deterministic and
  idempotent: same content -> same stamp, any asset edit -> new stamp sitewide.
- Wired as the final step of tools/build_site.py so every build/deploy stamps.
- Initial stamp applied across 259 pages + main.css (v=64a53dd6).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

en/student-welcome-guide/choose-a-programme/index.html

@@ -21,7 +21,7 @@
       gtag('js', new Date());
       gtag('config', 'G-WVYV807VSS');
     </script>
-    <script src="/consent.js" defer></script>
+    <script src="/consent.js?v=64a53dd6" defer></script>
     <meta name="description" content="Compare the programme first, then the city. Use information days well, and confirm language and aptitude-test requirements early.">
     <link rel="canonical" href="https://msosorg.com/en/student-welcome-guide/choose-a-programme/">
     <link rel="alternate" hreflang="en" href="https://msosorg.com/en/student-welcome-guide/choose-a-programme/">
@@ -59,7 +59,7 @@
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css" integrity="sha512-DTOQO9RWCH3ppGqcWaEA1BIZOC6xxalwEsw9c2QQeAIftl+Vegovlnee1c9QX4TctnWMn13TZye+giMm8e2LwA==" crossorigin="anonymous" referrerpolicy="no-referrer" media="print" onload="this.media=&#39;all&#39;" /><noscript><link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css" integrity="sha512-DTOQO9RWCH3ppGqcWaEA1BIZOC6xxalwEsw9c2QQeAIftl+Vegovlnee1c9QX4TctnWMn13TZye+giMm8e2LwA==" crossorigin="anonymous" referrerpolicy="no-referrer" /></noscript>
 
     <!-- Correct path to the main CSS file -->
-    <link rel="stylesheet" href="../../../css/main.css">
+    <link rel="stylesheet" href="../../../css/main.css?v=64a53dd6">
 </head>
 <body>
 
@@ -382,9 +382,9 @@
 </footer>
 <!-- END_FOOTER -->
 
-    <script src="../../../main.js"></script>
-    <script src="../../../guide-feedback.js"></script>
-<link rel="stylesheet" href="/assistant.css">
-<script src="/assistant.js" defer></script>
+    <script src="../../../main.js?v=64a53dd6"></script>
+    <script src="../../../guide-feedback.js?v=64a53dd6"></script>
+<link rel="stylesheet" href="/assistant.css?v=64a53dd6">
+<script src="/assistant.js?v=64a53dd6" defer></script>
 </body>
 </html>