feat: add box-drawing input support for treeView diagrams

- boxDrawingPreprocessor.ts: converts box-drawing chars to indent format

- parser.ts: integrate preprocessor with error line remapping

- 41 unit tests + 4 AST equivalence integration tests

- 2 demo sections in treeView.html

Author: notionparallax

demos/treeView.html

@@ -140,6 +140,44 @@ <h2>Folder Names with Spaces</h2>
       </div>
     </div>
 
+    <h2>Box-Drawing Input</h2>
+    <div class="demo">
+      <pre class="source"></pre>
+      <div class="rendered">
+        <pre class="mermaid">
+treeView-beta
+├── src/
+│   ├── components/
+│   │   ├── Button.tsx
+│   │   └── Modal.tsx
+│   ├── utils/
+│   │   └── helpers.ts
+│   └── index.js
+├── docs/
+│   └── README.md
+├── package.json
+└── .gitignore
+        </pre>
+      </div>
+    </div>
+
+    <h2>Box-Drawing with Annotations</h2>
+    <div class="demo">
+      <pre class="source"></pre>
+      <div class="rendered">
+        <pre class="mermaid">
+treeView-beta
+├── src/
+│   ├── App.tsx :::highlight ## main component
+│   ├── index.js ## entry point
+│   └── config.ts
+├── docs/
+│   └── guide.md ## user guide
+└── package.json
+        </pre>
+      </div>
+    </div>
+
     <h2>Custom Config</h2>
     <div class="demo">
       <pre class="source"></pre>