fix: Add Mermaid diagram rendering support to mdbook

Include mermaid.min.js and a custom init script that converts
```mermaid code blocks to rendered diagrams at runtime. Supports
theme-aware rendering (light/dark). No preprocessor needed.

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

book/book.toml

@@ -13,6 +13,7 @@ create-missing = false
 default-theme = "light"
 preferred-dark-theme = "navy"
 smart-punctuation = true
+additional-js = ["mermaid.min.js", "mermaid-init.js"]
 git-repository-url = "https://github.com/CloudNebulaProject/barycenter"
 edit-url-template = "https://github.com/CloudNebulaProject/barycenter/edit/main/book/{path}"
 site-url = "/"

book/mermaid-init.js

@@ -0,0 +1,57 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+(() => {
+    // Convert ```mermaid code blocks to <pre class="mermaid"> elements
+    const codeBlocks = document.querySelectorAll('code.language-mermaid');
+    codeBlocks.forEach((block) => {
+        const pre = block.parentElement;
+        const div = document.createElement('pre');
+        div.className = 'mermaid';
+        div.textContent = block.textContent;
+        pre.parentElement.replaceChild(div, pre);
+    });
+
+    if (codeBlocks.length === 0 && document.querySelectorAll('.mermaid').length === 0) {
+        return;
+    }
+
+    const darkThemes = ['ayu', 'navy', 'coal'];
+    const lightThemes = ['light', 'rust'];
+
+    const classList = document.getElementsByTagName('html')[0].classList;
+
+    let lastThemeWasLight = true;
+    for (const cssClass of classList) {
+        if (darkThemes.includes(cssClass)) {
+            lastThemeWasLight = false;
+            break;
+        }
+    }
+
+    const theme = lastThemeWasLight ? 'default' : 'dark';
+    mermaid.initialize({ startOnLoad: true, theme });
+
+    for (const darkTheme of darkThemes) {
+        const el = document.getElementById(darkTheme);
+        if (el) {
+            el.addEventListener('click', () => {
+                if (lastThemeWasLight) {
+                    window.location.reload();
+                }
+            });
+        }
+    }
+
+    for (const lightTheme of lightThemes) {
+        const el = document.getElementById(lightTheme);
+        if (el) {
+            el.addEventListener('click', () => {
+                if (!lastThemeWasLight) {
+                    window.location.reload();
+                }
+            });
+        }
+    }
+})();