seo: add description, canonical, Open Graph, JSON-LD, h1, sitemap and robots.txt

The published page had a title but no meta description, canonical URL,
Open Graph tags or document heading, and the site shipped no sitemap.xml
or robots.txt, so search engines had no declared canonical URL and no
sitemap to discover. Canonical points at the real page because the site
root (index.html) is a meta-refresh redirect.

Author: Keng0nion

Collatz reverse tree.html

@@ -4,6 +4,26 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no">
     <title>Collatz Reverse Tree – Interactive Visualizer with (m,k) toggle & PNG export</title>
+    <meta name="description" content="Explore the cycle-pruned reverse Collatz tree in your browser: adjust depth, click any node to trace its forward trajectory to 1, switch between integer and (m,k) coordinates, and export the fitted view as PNG.">
+    <link rel="canonical" href="https://keng0nion.github.io/Collatz-Conjecture-Reverse-Tree-Interactive-Visualizer/Collatz%20reverse%20tree.html">
+    <meta property="og:type" content="website">
+    <meta property="og:title" content="Collatz Reverse Tree – Interactive Visualizer with (m,k) toggle &amp; PNG export">
+    <meta property="og:description" content="Explore the cycle-pruned reverse Collatz tree in your browser: adjust depth, click any node to trace its forward trajectory to 1, switch between integer and (m,k) coordinates, and export the fitted view as PNG.">
+    <meta property="og:url" content="https://keng0nion.github.io/Collatz-Conjecture-Reverse-Tree-Interactive-Visualizer/Collatz%20reverse%20tree.html">
+    <meta property="og:image" content="https://keng0nion.github.io/Collatz-Conjecture-Reverse-Tree-Interactive-Visualizer/docs/screenshot.png">
+    <meta name="twitter:card" content="summary_large_image">
+    <script type="application/ld+json">
+    {
+      "@context": "https://schema.org",
+      "@type": "WebApplication",
+      "name": "Collatz Reverse Tree Visualizer",
+      "url": "https://keng0nion.github.io/Collatz-Conjecture-Reverse-Tree-Interactive-Visualizer/Collatz%20reverse%20tree.html",
+      "description": "An interactive browser visualizer for the cycle-pruned reverse Collatz tree, with adjustable depth, forward-trajectory highlighting, (m,k) coordinate labels, and PNG export.",
+      "applicationCategory": "EducationalApplication",
+      "isAccessibleForFree": true,
+      "inLanguage": "en"
+    }
+    </script>
     <!-- vis-network core library -->
     <script type="text/javascript" src="https://unpkg.com/vis-network@9.1.6/dist/vis-network.min.js"></script>
     <script type="text/javascript" src="collatz-math.js"></script>
@@ -101,6 +121,13 @@
             box-shadow: 0 1px 4px rgba(0,0,0,0.05);
             flex-shrink: 1;
         }
+        .info-panel h1 {
+            display: inline;
+            margin: 0;
+            font-size: 1em;
+            font-weight: 700;
+            color: #e67e22;
+        }
         .info-panel strong {
             color: #e67e22;
         }
@@ -199,7 +226,7 @@
         <button id="toggleDisplayBtn" class="toggle-btn">🔄 Toggle (m,k) Coordinates</button>
         <button id="exportPngBtn" class="export-btn">📸 Export PNG (Fitted View)</button>
         <div class="info-panel" id="infoPanel">
-            📘 <strong>Cycle-pruned Collatz Reverse Tree</strong> · growing backwards from 1<br>
+            📘 <h1>Cycle-pruned Collatz Reverse Tree</h1> · growing backwards from 1<br>
             🟢 Click any node to show its forward trajectory to 1 · Switch to (m,k) mode
         </div>
         <div class="stats" id="statsArea">

robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://keng0nion.github.io/Collatz-Conjecture-Reverse-Tree-Interactive-Visualizer/sitemap.xml

sitemap.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://keng0nion.github.io/Collatz-Conjecture-Reverse-Tree-Interactive-Visualizer/Collatz%20reverse%20tree.html</loc>
+    <lastmod>2026-10-13</lastmod>
+  </url>
+</urlset>