feat: add merge_all_wren utility, strutil demo, and 7 new API manual pages

Add WrenFileReader class in apps/merge_all_wren.wren for recursive .wren file discovery and content printing. Introduce example/strutil_demo.wren demonstrating case conversion, hex encoding, SHA256 hashing, string repetition, padding, HTML/JSON escaping, and URL encoding/decoding. Create manual/api pages for argparse, dataset, html, markdown, uuid, wdantic, and web modules. Update manual/api/index.html sidebar and module count from 21 to 28, adding links and cards for the new modules.

test/markdown/lists_advanced.wren

@@ -0,0 +1,22 @@
+// retoor <retoor@molodetz.nl>
+
+import "markdown" for Markdown
+
+var star = Markdown.toHtml("* item a\n* item b")
+System.print(star.contains("<ul>")) // expect: true
+System.print(star.contains("<li>item a</li>")) // expect: true
+System.print(star.contains("<li>item b</li>")) // expect: true
+
+var plus = Markdown.toHtml("+ item x\n+ item y")
+System.print(plus.contains("<ul>")) // expect: true
+System.print(plus.contains("<li>item x</li>")) // expect: true
+System.print(plus.contains("<li>item y</li>")) // expect: true
+
+var multiDigit = Markdown.toHtml("10. tenth\n11. eleventh")
+System.print(multiDigit.contains("<ol>")) // expect: true
+System.print(multiDigit.contains("<li>tenth</li>")) // expect: true
+System.print(multiDigit.contains("<li>eleventh</li>")) // expect: true
+
+var formatted = Markdown.toHtml("- **bold item**\n- *italic item*")
+System.print(formatted.contains("<li><strong>bold item</strong></li>")) // expect: true
+System.print(formatted.contains("<li><em>italic item</em></li>")) // expect: true