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/mixed_content.wren

@@ -0,0 +1,40 @@
+// retoor <retoor@molodetz.nl>
+
+import "markdown" for Markdown
+
+var doc = "# Title
+
+This is a paragraph with **bold** text.
+
+- Item 1
+- Item 2
+
+Another paragraph.
+
+> A quote
+
+---
+
+## Subtitle
+
+1. First
+2. Second"
+
+var html = Markdown.toHtml(doc)
+System.print(html.contains("<h1>Title</h1>")) // expect: true
+System.print(html.contains("<strong>bold</strong>")) // expect: true
+System.print(html.contains("<ul>")) // expect: true
+System.print(html.contains("<li>Item 1</li>")) // expect: true
+System.print(html.contains("<blockquote>")) // expect: true
+System.print(html.contains("<hr>")) // expect: true
+System.print(html.contains("<h2>Subtitle</h2>")) // expect: true
+System.print(html.contains("<ol>")) // expect: true
+System.print(html.contains("<li>First</li>")) // expect: true
+
+var linkInList = Markdown.toHtml("- [Link](http://example.com)")
+System.print(linkInList.contains("<li>")) // expect: true
+System.print(linkInList.contains("<a href=\"http://example.com\">Link</a>")) // expect: true
+
+var codeInHeading = Markdown.toHtml("# Using `print()`")
+System.print(codeInHeading.contains("<h1>")) // expect: true
+System.print(codeInHeading.contains("<code>print()</code>")) // expect: true