Add HLD/LLD architecture page and clarify agent-framework vs story-tree branching

Adds docs/architecture.html (a standalone walkthrough of the context
budget, world-state engine, story tree, egress fix, and pen-test
findings) and links it from the landing page. Also clarifies in
GUIDE.md §1.8 that "no branching" describes per-turn control flow,
not the story tree's branching data.

docs/index.html

@@ -131,6 +131,7 @@
     <div class="cta">
       <a class="btn btn-primary" href="https://ai-dnd-1gmp.onrender.com">Launch the live demo →</a>
       <a class="btn btn-ghost" href="guide.html">Read the design notes</a>
+      <a class="btn btn-ghost" href="architecture.html">HLD &amp; LLD walkthrough</a>
       <a class="btn btn-ghost" href="https://github.com/parththakkar106/AI-DnD">View the source</a>
     </div>
     <p class="wake">No sign-up, no API key. Hosted on a free tier that sleeps — the first load takes ~30–60s to wake.</p>